**2. Solutions for increasing the adherence at the titanium substrate / glass coating interface**

It is widely accepted that the integrity of the substrate/coating interface is always critical in determining the performance and the reliability of any implant-type coating. Generally, low values of adhesion for bioglass coatings were published (Mardare et al., 2003; Goller, 2004; Peddi et al., 2008). Among the deposition techniques available for producing bioglass coatings, magnetron sputtering is the less explored. Only three papers have been published by other groups on this topic to the best of our knowledge (Mardare et al., 2003; Wolke et al., 2008; Saino et al., 2010). The main impediment in using bioglass coatings as implant

Magnetron Sputtered BG Thin Films: An Alternative Biofunctionalization

Approach – Peculiarities of Bioglass Sputtering and Bioactivity Behaviour 75

In case of the as-deposited and 550°C annealed structures the failure always occurs in glue's volume without damaging the film integrity at an average value of 85 MPa. This represents the bonding limit of the resin, as confirmed by the manufacturer. For the structures annealed at 750°C the adherence was estimated at 72.9±7.1 MPa, the film being detached each time. However, the measured film-substrate adhesion values are much higher than

Fig. 1. GIXRD patterns collected for the BG1 films before and after the post-deposition heattreatments: - titanium alloy substrate; - Na2Mg(PO3)4; - CaTiO3; - TiO2-rutile;

These high values of pull-out strength for the as-deposited coatings, and those annealed at 550°C, are attributed to the sputter cleaning and ion bombarding processes of the substrate, done before deposition (G.E. Stan et al., 2009). As it is known, the poor adhesion of the coatings could be ascribed to the natural oxide layers present on the titanium alloy surface prior deposition. An optimal solution for removing these contaminants in order to increase the adherence is an argon plasma etching pre-treatment of the titanium substrates. The sputter cleaning process largely removes not only the native oxide layers, but also the adsorbed gas molecules, to produce a clean, highly active surface (Lacefield, 1988). The argon ion bombarding process during sputtering might enhance the atomic diffusion and mixing in the near-interface region. The etching process has been optimized by tuning plasma power (~200 W), surface DC bias (0.4 kV) and etching time (10 min), based on own experience and literature (Mattox, 1994). The sputtering time offers a maximum of the adhesion properties after 10 min, as a result of the removal of surface oxides and contaminants, which leads to good bonding at

The decrease of the adherence after heat-treatment at 750°C, is due the BG layers/titanium substrate dilatation coefficients misfit at high temperatures and to the strong oxidation


the coating-substrate interface (G.E. Stan et al., 2009).

those reported in literature (Mardare et al., 2003; Goller, 2004; Peddi et al., 2008).

applications is the high thermal expansion coefficients of bioglass, about 12–17 x 10-6/°C, relative to that of medical-grade titanium ~9.2 x 10-6/°C (L.L. Hench & J. Wilson, 2003; Goller, 2004).

The adherence of the substrate/coating interface is always considered when estimating the implant-type coating reliability in medical practice. The mechanical quality of the interface can be evaluated by pull-out strength measurements. The pull-out measurements were carried out using an adhesion tester – DFD Instruments PAT MICRO adhesion tester AT101 (maximum pull force = 1 kN) equipped with Φ 2.8 mm stainless steel test elements. The test elements were glued to the film's surface with a cyanoacrylate one-component Epoxy adhesive, E1100S. The stub surface was first polished, ultrasonically degreased in acetone and ethanol and dried in a nitrogen flow. After gluing, the samples were placed in an oven for thermal curing (130°C/1 h). Each test element was pulled-out vertically with a calibrated hydraulic pump until detachment. The experimental procedure was conducted in accordance with the ASTM D4541 and ISO 4624 standards. The adhesion strength was determined from the recorded failure value divided by the quantified detached surface area. Mean values and standard deviations were computed. The statistical significance was determined using an unpaired Student's t–test. The differences were considered significant when p < 0.05.

In a first step, a bioglass (BG1) mild-pressed powder having the following composition (wt %): SiO2 - 55, CaO - 15, P2O5 - 10, K2O - 10, MgO - 5, Na2O – 5, having a low CTE (~ 10.2 x 10-6/°C) was used as cathode target. The BG1 samples preparation details are presented in *Table 1*.


Table 1. BG1 sputtering deposition conditions and additional sample preparation details

The as-deposited structures as well as the heat-treated ones were investigated. *Figure 1* presents the GIXRD patterns of the BG1 structures before and after the devitrification heat-treatments. The structure of the as-deposited film is amorphous at the sensitivity limit of the measurement. The amorphous component is probably based on amorphous silica, as it can be deduced from the main hump centred on 2θ≈22°. After heat treatment at 550°C, several weak TiO2-rutile – ICDD: 12-1276 lines appear, whose intensity increases by annealing at 750°C. During heat treatment at 750°C, crystallization occurs in the layer, but the amorphous component only slightly diminishes. The few rather intensive peaks which appear, were assigned to: Na2Mg(PO3)4 – ICDD: 22-477 and TiO2 rutile. There are also several weak lines that might be associated with small percents of perovskite (CaTiO3 – ICDD: 78-1013) and oxygen deficient titanium oxides (Ti2O – ICDD: 11-0218 and TiO – ICDD: 72-0020). The formation of calcium titanate and, at least partly, of titanium oxides in the heat-treated samples, is due to the inter-diffusion at the filmsubstrate interface. The presence of perovskite in heat-treated hydroxyapatite films deposited onto titanium had already been observed by other authors (Wei et al., 2005; Berezhnaya et al., 2008).

applications is the high thermal expansion coefficients of bioglass, about 12–17 x 10-6/°C, relative to that of medical-grade titanium ~9.2 x 10-6/°C (L.L. Hench & J. Wilson, 2003;

The adherence of the substrate/coating interface is always considered when estimating the implant-type coating reliability in medical practice. The mechanical quality of the interface can be evaluated by pull-out strength measurements. The pull-out measurements were carried out using an adhesion tester – DFD Instruments PAT MICRO adhesion tester AT101 (maximum pull force = 1 kN) equipped with Φ 2.8 mm stainless steel test elements. The test elements were glued to the film's surface with a cyanoacrylate one-component Epoxy adhesive, E1100S. The stub surface was first polished, ultrasonically degreased in acetone and ethanol and dried in a nitrogen flow. After gluing, the samples were placed in an oven for thermal curing (130°C/1 h). Each test element was pulled-out vertically with a calibrated hydraulic pump until detachment. The experimental procedure was conducted in accordance with the ASTM D4541 and ISO 4624 standards. The adhesion strength was determined from the recorded failure value divided by the quantified detached surface area. Mean values and standard deviations were computed. The statistical significance was determined using an unpaired Student's t–test. The differences were considered significant

In a first step, a bioglass (BG1) mild-pressed powder having the following composition (wt %): SiO2 - 55, CaO - 15, P2O5 - 10, K2O - 10, MgO - 5, Na2O – 5, having a low CTE (~ 10.2 x 10-6/°C) was used as cathode target. The BG1 samples preparation details are presented in

BG1 0.3 Ar Ti6Al7Nb 750 nm 550°C/2h in air

Table 1. BG1 sputtering deposition conditions and additional sample preparation details

The as-deposited structures as well as the heat-treated ones were investigated. *Figure 1* presents the GIXRD patterns of the BG1 structures before and after the devitrification heat-treatments. The structure of the as-deposited film is amorphous at the sensitivity limit of the measurement. The amorphous component is probably based on amorphous silica, as it can be deduced from the main hump centred on 2θ≈22°. After heat treatment at 550°C, several weak TiO2-rutile – ICDD: 12-1276 lines appear, whose intensity increases by annealing at 750°C. During heat treatment at 750°C, crystallization occurs in the layer, but the amorphous component only slightly diminishes. The few rather intensive peaks which appear, were assigned to: Na2Mg(PO3)4 – ICDD: 22-477 and TiO2 rutile. There are also several weak lines that might be associated with small percents of perovskite (CaTiO3 – ICDD: 78-1013) and oxygen deficient titanium oxides (Ti2O – ICDD: 11-0218 and TiO – ICDD: 72-0020). The formation of calcium titanate and, at least partly, of titanium oxides in the heat-treated samples, is due to the inter-diffusion at the filmsubstrate interface. The presence of perovskite in heat-treated hydroxyapatite films deposited onto titanium had already been observed by other authors (Wei et al., 2005;

Substrate type

Film thickness

Post-dep. heat-treatments

750°C/2h in air

Working atmosphere

Goller, 2004).

when p < 0.05.

*Table 1*.

Bioglass target

Berezhnaya et al., 2008).

Sputtering pressure (Pa) In case of the as-deposited and 550°C annealed structures the failure always occurs in glue's volume without damaging the film integrity at an average value of 85 MPa. This represents the bonding limit of the resin, as confirmed by the manufacturer. For the structures annealed at 750°C the adherence was estimated at 72.9±7.1 MPa, the film being detached each time. However, the measured film-substrate adhesion values are much higher than those reported in literature (Mardare et al., 2003; Goller, 2004; Peddi et al., 2008).

Fig. 1. GIXRD patterns collected for the BG1 films before and after the post-deposition heattreatments: - titanium alloy substrate; - Na2Mg(PO3)4; - CaTiO3; - TiO2-rutile; - Ti2O; - TiO

These high values of pull-out strength for the as-deposited coatings, and those annealed at 550°C, are attributed to the sputter cleaning and ion bombarding processes of the substrate, done before deposition (G.E. Stan et al., 2009). As it is known, the poor adhesion of the coatings could be ascribed to the natural oxide layers present on the titanium alloy surface prior deposition. An optimal solution for removing these contaminants in order to increase the adherence is an argon plasma etching pre-treatment of the titanium substrates. The sputter cleaning process largely removes not only the native oxide layers, but also the adsorbed gas molecules, to produce a clean, highly active surface (Lacefield, 1988). The argon ion bombarding process during sputtering might enhance the atomic diffusion and mixing in the near-interface region. The etching process has been optimized by tuning plasma power (~200 W), surface DC bias (0.4 kV) and etching time (10 min), based on own experience and literature (Mattox, 1994). The sputtering time offers a maximum of the adhesion properties after 10 min, as a result of the removal of surface oxides and contaminants, which leads to good bonding at the coating-substrate interface (G.E. Stan et al., 2009).

The decrease of the adherence after heat-treatment at 750°C, is due the BG layers/titanium substrate dilatation coefficients misfit at high temperatures and to the strong oxidation

Magnetron Sputtered BG Thin Films: An Alternative Biofunctionalization

thickness of ~ 700 nm (BG2-G).

magnetron co-sputtering

between the substrate and the film.

Approach – Peculiarities of Bioglass Sputtering and Bioactivity Behaviour 77

In such a case, in the attempt to increase the adherence properties of the BG2 film to the titanium substrate, a buffer layer with chemical gradient of composition was introduced. The graded BG1-xTix/Ti (x=0-1) structure was prepared by a slow continuous shifting of the rotating substrate holder during the co-sputtering deposition, from the Ti target towards the BG one (G.E. Stan et al., 2010c). This way, a functionally graded transition zone with a variable chemical composition was forming between the BG biofunctional coating and the Ti substrate (*Fig. 3*). This process lasted for 5 minutes, the graded layer thickness being estimated at ~70 nm. Next, the formed graded structure was placed in front of the BG target and the sputtering continued for 1 hour in order to deposit the functional BG layer with a

Fig. 3. Schematic diagram of the graded BG1-xTix/Ti (x=0-1) structure deposition by

For bonding strength comparison, we synthesized by RF-MS, under identical experimental conditions, 700 nm BG/Ti "abrupt coatings" (BG2-A) without the intermediate buffer layer

As an improvement of the mechanical and biological properties of films is known to be achieved by the transformation of BG into glass-ceramics via heat-treatments (Peitl Filho et al., 1996; El Batal et al., 2003), we have chosen for our study an annealing temperature of 650ºC/2h in order to induce the partial crystallization of the combeite (Na2CaSi2O6 ) phase. As known (Lefebvre et al., 2007), the crystallization of the combeite is reached within the range (610 - 700ºC) and the larger the temperature, the higher the crystallization degree. Low heating and cooling rates (1°C/min) have been applied in order to minimize the residual mechanical stress in films at the end of the thermal cycle (Berbecaru et al., 2010).

phenomena which leaded to the of needle-shaped rutile crystals agglomerates which penetrate through the film (*Fig. 2*).

On the other hand, the formation during the heat-treatments of a perovskite-type phase (CaTiO3) suggested an inter-diffusion phenomenon between the coating and the substrate during heat-treatment, which appears to be the predominant factor in determining a film adhesion even in case of high temperature annealing treatments. The formation of various types of titanium oxides and sub-oxides is also in agreement with this hypothesis. This finding could be further exploited by designing special heat-treatments which would lead to nucleation at the interface of inter-mix BG-Ti phases with role in strengthening the coating adherence.

Fig. 2. SEM images of a BG1 film heat-treated in air at 750°C/2h

The classical 45S5 composition system (SiO2 – 45 wt%, CaO – 24.5 wt%, P2O5 – 6 wt%, Na2O – 24.5 wt%), patented by Hench a couple of decades ago, has a significantly higher thermal expansion coefficient (15 – 17 x 10-6/°C) than the titanium and titanium alloys materials. 45S5 commercial powders have been mild-pressed to prepared cathode targets (BG2). The BG2 samples preparation details are presented in *Table 2*.


Table 2. BG2 sputtering deposition conditions and additional sample preparation details

phenomena which leaded to the of needle-shaped rutile crystals agglomerates which

On the other hand, the formation during the heat-treatments of a perovskite-type phase (CaTiO3) suggested an inter-diffusion phenomenon between the coating and the substrate during heat-treatment, which appears to be the predominant factor in determining a film adhesion even in case of high temperature annealing treatments. The formation of various types of titanium oxides and sub-oxides is also in agreement with this hypothesis. This finding could be further exploited by designing special heat-treatments which would lead to nucleation at the

The classical 45S5 composition system (SiO2 – 45 wt%, CaO – 24.5 wt%, P2O5 – 6 wt%, Na2O – 24.5 wt%), patented by Hench a couple of decades ago, has a significantly higher thermal expansion coefficient (15 – 17 x 10-6/°C) than the titanium and titanium alloys materials. 45S5 commercial powders have been mild-pressed to prepared cathode targets (BG2). The

> Working atmosphere

0.3 Pa Ar cp-Ti

0.3 Pa Ar cp-Ti

Table 2. BG2 sputtering deposition conditions and additional sample preparation details

Substrate type

grade 4

grade 4

Film thickness

> ~ 700 nm

> ~ 770 nm

Post-dep. heattreatments

650°C/2h in air

650°C/2h in air

interface of inter-mix BG-Ti phases with role in strengthening the coating adherence.

Fig. 2. SEM images of a BG1 film heat-treated in air at 750°C/2h

Sputtering pressure

BG2 samples preparation details are presented in *Table 2*.

Sample type

Simple BG2-A (BG/Ti)

Graded BG2-G (BG/BG1 xTix/Ti)

Bioglass target

BG2

BG2

penetrate through the film (*Fig. 2*).

In such a case, in the attempt to increase the adherence properties of the BG2 film to the titanium substrate, a buffer layer with chemical gradient of composition was introduced. The graded BG1-xTix/Ti (x=0-1) structure was prepared by a slow continuous shifting of the rotating substrate holder during the co-sputtering deposition, from the Ti target towards the BG one (G.E. Stan et al., 2010c). This way, a functionally graded transition zone with a variable chemical composition was forming between the BG biofunctional coating and the Ti substrate (*Fig. 3*). This process lasted for 5 minutes, the graded layer thickness being estimated at ~70 nm. Next, the formed graded structure was placed in front of the BG target and the sputtering continued for 1 hour in order to deposit the functional BG layer with a thickness of ~ 700 nm (BG2-G).

Fig. 3. Schematic diagram of the graded BG1-xTix/Ti (x=0-1) structure deposition by magnetron co-sputtering

For bonding strength comparison, we synthesized by RF-MS, under identical experimental conditions, 700 nm BG/Ti "abrupt coatings" (BG2-A) without the intermediate buffer layer between the substrate and the film.

As an improvement of the mechanical and biological properties of films is known to be achieved by the transformation of BG into glass-ceramics via heat-treatments (Peitl Filho et al., 1996; El Batal et al., 2003), we have chosen for our study an annealing temperature of 650ºC/2h in order to induce the partial crystallization of the combeite (Na2CaSi2O6 ) phase. As known (Lefebvre et al., 2007), the crystallization of the combeite is reached within the range (610 - 700ºC) and the larger the temperature, the higher the crystallization degree. Low heating and cooling rates (1°C/min) have been applied in order to minimize the residual mechanical stress in films at the end of the thermal cycle (Berbecaru et al., 2010).

Magnetron Sputtered BG Thin Films: An Alternative Biofunctionalization

thermal expansion coefficients (G.E. Stan et al., 2010c).

Sputtering pressure

BG3 BG3-1 0.2 Pa 100%Ar cp-Ti

BG3 BG3-2 0.3 Pa 100%Ar cp-Ti

BG3 BG3-3 0.4 Pa 100%Ar cp-Ti

BG3 BG3-4 0.3 Pa 93%Ar+7%O2

Bioglass target

Sample denomination

BG3-2 and BG3-3 (G.E. Stan et al., 2011).

Approach – Peculiarities of Bioglass Sputtering and Bioactivity Behaviour 79

A 1.7 times higher bonding strength (50.3 ± 5.8 MPa) was obtained in case of the BG2-G structure (G.E. Stan et al., 2010c). The bonding strength to Ti substrates characteristic to heat-treated BG2-G samples is larger than current values reported in literature (50 MPa vs. 30 MPa). The BG2-G design eliminates the material interface discontinuity due to the formation of a BGxTi1-x (x=0-1) functionally graded buffer layer that improves the bonding strength. The result indicated that the synthesis of BG structures with graded buffer layers is a feasible solution for preparing adherent BG coatings, even in case of bioglasses with high

In the following paragraphs results will be presented depicting the influence of typical sputtering variables (deposition pressure, working gas composition) on the BG thin films adherence. For these studies a novel complex bioglass powder composition (BG3) was chosen as cathode target (wt %): SiO2 – 40.08, CaO – 29.1, MgO – 8.96, P2O5 – 6.32, CaF2 –

5.79, B2O3 – 5.16, and Na2O – 4.59. The deposition conditions are presented in *Table 3*.

Working atmosphere composition

Table 3. BG3 sputtering deposition conditions and additional sample preparation details

The GIXRD measurements evidenced the amorphous state of all as-deposited BG3 thin films. *Figure 5* displays the SEM micrographs of the as-deposited BG films. One can see important modifications of the morphology at a sub-micrometric level when varying the deposition conditions. The SEM micrographs revealed well-adhered films with a homogeneous surface microstructure for all the as-deposited samples. No signs of micro-cracks or delaminations were noticed. The rough microstructure, consisting in parallel alternant, stripe-like tall regions, delimited by narrow depressions was probably induced by titanium substrate, while the fine structure consists of nano-sized merged-granules (*Fig. 5 - inset*). One can not see significant influence of pressure upon the microstructure (BG3-1, BG3-2 and BG3-3). In case of reactive atmosphere (BG3-4) the films' surfaces presented interesting features. The coating is uniformly covered by tower-shaped nano-formations with an average diameter of ~70 nm. The towershaped nano-aggregates seem to be ingrowths nucleated on a matrix which is similar to BG3-1,

The physics of the magnetron sputtering process at different working pressures and compositions of the working atmosphere determines the thickness and morphology of the as-deposited BG films. The films' thicknesses varied between 380 and 646 nm, the thicker for the lowest pressure non-reactive deposition atmosphere. The decrease of the film thickness with the increase of Ar pressure from 0.2 to 0.4 Pa might be assigned to a decreased fraction

Substrate type

cp-Ti

Deposition time

grade 1 70 min 646 nm

grade 1 70 min 510 nm

grade 1 70 min 480 nm

grade 1 70 min 380 nm

Film thickness

The GIXRD analysis of the BG2-G coating before heat treatment confirmed the amorphous nature of the deposited films (*Fig. 4*). The titanium sub-oxide phase, Ti3O (ICDD: 73-1583) was present in the structure of the untreated sample. The structure was crystallized after heat-treatment with combeite - ICDD: 75-1687, CaSiO3 (wollastonite) - ICDD: 42–550, and Na3PO4 - ICDD: 76-202 as main crystalline phases. There was noticed a strong signal originating from the Ti substrate - ICDD: 44-1294. After the heat-treatment, Ti lines were shifted towards lower angles, simultaneously with a strong broadening. Moreover, the Ti sub-oxide completely disappeared and was replaced by two well crystallized Ti dioxide phases, anatase - ICDD: 89-4203 and rutile - ICDD: 12-1276 (G.E. Stan et al., 2010c).

Fig. 4. GIXRD patterns collected for the BG2 films before and after the post-deposition heattreatments: - titanium substrate; - combeite (Na2CaSi2O6 ); - wollastonite (CaSiO3); ■- Na3PO4;- TiO2-anatase; - TiO2-rutile; - Ti3O

The partial crystallization of the BG structure may prove advantageous for biomedical applications as it results in improved bioactivity due to formation on their surface of calcium phosphate (CaP) rich layers in contact with simulated body media, as would be demonstrated in the Subsection 3.3. The formation of such an apatite type layer is very important for bone growth and bonding ability.

A statistical analysis was performed based on the average value for ten different BG samples in case of each type of structure. For the BG2-A structure, the bonding failure occurred at a mean value of 29.2 ± 7 MPa (G.E. Stan et al., 2010c). This is a rather low bonding strength value, but similar to those reported in literature (Mardare et al., 2003; Goller, 2004; Peddi et al., 2008). This effect is mainly due to the significant difference between the thermal expansion coefficients of the BG film and the titanium substrate at high temperatures.

The GIXRD analysis of the BG2-G coating before heat treatment confirmed the amorphous nature of the deposited films (*Fig. 4*). The titanium sub-oxide phase, Ti3O (ICDD: 73-1583) was present in the structure of the untreated sample. The structure was crystallized after heat-treatment with combeite - ICDD: 75-1687, CaSiO3 (wollastonite) - ICDD: 42–550, and Na3PO4 - ICDD: 76-202 as main crystalline phases. There was noticed a strong signal originating from the Ti substrate - ICDD: 44-1294. After the heat-treatment, Ti lines were shifted towards lower angles, simultaneously with a strong broadening. Moreover, the Ti sub-oxide completely disappeared and was replaced by two well crystallized Ti dioxide

phases, anatase - ICDD: 89-4203 and rutile - ICDD: 12-1276 (G.E. Stan et al., 2010c).

Fig. 4. GIXRD patterns collected for the BG2 films before and after the post-deposition heattreatments: - titanium substrate; - combeite (Na2CaSi2O6 ); - wollastonite (CaSiO3);

The partial crystallization of the BG structure may prove advantageous for biomedical applications as it results in improved bioactivity due to formation on their surface of calcium phosphate (CaP) rich layers in contact with simulated body media, as would be demonstrated in the Subsection 3.3. The formation of such an apatite type layer is very

A statistical analysis was performed based on the average value for ten different BG samples in case of each type of structure. For the BG2-A structure, the bonding failure occurred at a mean value of 29.2 ± 7 MPa (G.E. Stan et al., 2010c). This is a rather low bonding strength value, but similar to those reported in literature (Mardare et al., 2003; Goller, 2004; Peddi et al., 2008). This effect is mainly due to the significant difference between the thermal expansion coefficients of the BG film and the titanium substrate at

■- Na3PO4;- TiO2-anatase; - TiO2-rutile; - Ti3O

important for bone growth and bonding ability.

high temperatures.

A 1.7 times higher bonding strength (50.3 ± 5.8 MPa) was obtained in case of the BG2-G structure (G.E. Stan et al., 2010c). The bonding strength to Ti substrates characteristic to heat-treated BG2-G samples is larger than current values reported in literature (50 MPa vs. 30 MPa). The BG2-G design eliminates the material interface discontinuity due to the formation of a BGxTi1-x (x=0-1) functionally graded buffer layer that improves the bonding strength. The result indicated that the synthesis of BG structures with graded buffer layers is a feasible solution for preparing adherent BG coatings, even in case of bioglasses with high thermal expansion coefficients (G.E. Stan et al., 2010c).

In the following paragraphs results will be presented depicting the influence of typical sputtering variables (deposition pressure, working gas composition) on the BG thin films adherence. For these studies a novel complex bioglass powder composition (BG3) was chosen as cathode target (wt %): SiO2 – 40.08, CaO – 29.1, MgO – 8.96, P2O5 – 6.32, CaF2 – 5.79, B2O3 – 5.16, and Na2O – 4.59. The deposition conditions are presented in *Table 3*.


Table 3. BG3 sputtering deposition conditions and additional sample preparation details

The GIXRD measurements evidenced the amorphous state of all as-deposited BG3 thin films. *Figure 5* displays the SEM micrographs of the as-deposited BG films. One can see important modifications of the morphology at a sub-micrometric level when varying the deposition conditions. The SEM micrographs revealed well-adhered films with a homogeneous surface microstructure for all the as-deposited samples. No signs of micro-cracks or delaminations were noticed. The rough microstructure, consisting in parallel alternant, stripe-like tall regions, delimited by narrow depressions was probably induced by titanium substrate, while the fine structure consists of nano-sized merged-granules (*Fig. 5 - inset*). One can not see significant influence of pressure upon the microstructure (BG3-1, BG3-2 and BG3-3). In case of reactive atmosphere (BG3-4) the films' surfaces presented interesting features. The coating is uniformly covered by tower-shaped nano-formations with an average diameter of ~70 nm. The towershaped nano-aggregates seem to be ingrowths nucleated on a matrix which is similar to BG3-1, BG3-2 and BG3-3 (G.E. Stan et al., 2011).

The physics of the magnetron sputtering process at different working pressures and compositions of the working atmosphere determines the thickness and morphology of the as-deposited BG films. The films' thicknesses varied between 380 and 646 nm, the thicker for the lowest pressure non-reactive deposition atmosphere. The decrease of the film thickness with the increase of Ar pressure from 0.2 to 0.4 Pa might be assigned to a decreased fraction

Magnetron Sputtered BG Thin Films: An Alternative Biofunctionalization

between the results obtained for the different coatings (p<0.05).

effect" (G.E. Stan et al., 2011).

**3.1 As-deposited films analysis** 

exhibiting an enhanced bioactivity.

et al., 2004).

Approach – Peculiarities of Bioglass Sputtering and Bioactivity Behaviour 81

2004; Peddi et al., 2008). For the films deposited at higher argon pressure (BG3-3), the adherence dramatically decreased to a mean value of 34.2 ± 12.0 MPa, the bonding failure occurring at the film - substrate interface. When using a reactive atmosphere (BG3-4), but keeping the total pressure constant at 0.3 Pa, the adhesion strength of monolithic BG coating declined down to 44.0 ± 6.8 MPa (G.E. Stan et al., 2011). Similar adherence values have been generally reported in literature (Mardare et al., 2003; Goller, 2004; Peddi et al., 2008). The two tailed t-testing, assuming unequal variances, showed statistically significant differences

The excellent adherence value of BG3-1 and BG3-2 films must be emphasized. The adherence values are significantly higher than those reported in literature for this kind of implant coatings, thereby opening new perspectives in implantology. The high adherence is related to the processes characteristic of magnetron plasma sputtering. At a lower pressure the sputtered atoms collide with the substrate with higher kinetic energy, creating the possibility of forming chemical bonds with atoms from the substrate or being implanted into substrate. Such phenomena could lead to an increased adherence in case of the BG3-1 and BG3-2 films. Using a higher pressure (BG3-3) results in a spatial density variation of the background gas and affects the magnetron sputtering discharge as well as the transport of the particles towards the substrate. The energy flux at the substrate is thus affected, which in turn affects the properties of the growing film such as density, grain size, columnar structure, stoichiometry, coverage and adhesion (G.E. Stan et al., 2011). Moreover, their surface mobility is dramatically reduced, causing possible film inhomogeneities, such as voids or clustering hillocks (G.E. Stan et al., 2010d). In case of BG3-4 films, the presence of the tower shaped glassy nano-aggregates on the BG surface could be the underlying reason for the decreased adherence. Usually when dealing with tensile forces, rupture involves only a few molecules in the material causing the whole specimen to fracture in a "domino

**3. Bioactivity tailoring of bioglass and glass-ceramic sputtered thin films** 

An implant-type ideal coating should constitute a proper mechanical support while

The bioglass structure is very complex; there is only short and medium range order determined by chemical bounding and steric hindrance. The silica-based glass structure is generally viewed as a matrix composed of SiO4 tetrahedra connected at the corners to form a continuous tri-dimensional network with all bridging oxygen (BOs). The SiO4 tetrahedra network is slightly distorted due to variations in the bond angles and the torsion angles. The network modifiers (alkali and alkali-earth ions typical to bioglasses) enter the structure as singly or double charged cations and occupy interstitial sites. Their charge is compensated by non-bridging oxygen bonds (NBOs), created by breaking bridges between adjacent SiO4 tetrahedra. The increase of modifier content generates the creation of large NBOs concentrations, reducing the connectivity of the BG network, with direct effect upon electrical conduction, the thermal expansion coefficient, glass transition temperature, chemical corrosion in aqueous media and reactivity (Serra et al., 2002; Liste

of sputtered particles reaching the substrate, due to the increased probability of collision with other particles when running from target to substrate. But, despite the masses of the sputtered atoms (Ms) in case of BG material are comparable to that of the background Ar gas (MAr/Ms= 1–1.8 ), thus favouring the kinetic energy loss by collision, the thermalization or removal while running towards the substrate is unlike because of the short target-tosubstrate distance (only 30 mm) (Palmero et al., 2007, G.E. Stan et al., 2010d).

Under these deposition conditions it is highly probable that the main phenomenon leading to decreasing growth rate of BG films is the occurrence of charge transfer reactions in Ar (van Hattum et al., 2007). These processes lead to possible modifications of the energy and extent of the argon ion and neutral bombardment during the deposition in the considered pressure region. The occurrence of resonant charge transfer reactions is known to lower the energy of bombarding ions, determining significant variations of the sputter yield.

Fig. 5. SEM images of as-deposited BG3 sputtered films

The decreased deposition rate observed in the presence of oxygen in the working atmosphere can be attributed to "target poisoning" induced by chemisorption and oxygen ion implantation (Berg & Nyberg, 2005). During the sputtering process the BG target is bombarded by ions from the plasma, including reactive oxygen ions. This leads to the formation of a compound film not only on the substrate as desired, but also on the sputtering target. This results in a significantly reduced sputter yield, and thereby, a reduced deposition rate (G.E. Stan et al., 2010a).

Significant pull-out adherence differences were found function of sputtering deposition regime. In the case of BG3-1 and BG3-2 films, the failure occurred in the epoxy adhesive's volume at 84.8 ± 1.5 MPa without damaging the film integrity (G.E. Stan et al., 2011). As this value represents the bonding limit of the epoxy adhesive as confirmed by the manufacturer, the true BG coating – Ti substrate bonding strength could be even higher. This adhesion value is much higher than the usual ones reported in literature (Mardare et al., 2003; Goller,

of sputtered particles reaching the substrate, due to the increased probability of collision with other particles when running from target to substrate. But, despite the masses of the sputtered atoms (Ms) in case of BG material are comparable to that of the background Ar gas (MAr/Ms= 1–1.8 ), thus favouring the kinetic energy loss by collision, the thermalization or removal while running towards the substrate is unlike because of the short target-to-

Under these deposition conditions it is highly probable that the main phenomenon leading to decreasing growth rate of BG films is the occurrence of charge transfer reactions in Ar (van Hattum et al., 2007). These processes lead to possible modifications of the energy and extent of the argon ion and neutral bombardment during the deposition in the considered pressure region. The occurrence of resonant charge transfer reactions is known to lower the

The decreased deposition rate observed in the presence of oxygen in the working atmosphere can be attributed to "target poisoning" induced by chemisorption and oxygen ion implantation (Berg & Nyberg, 2005). During the sputtering process the BG target is bombarded by ions from the plasma, including reactive oxygen ions. This leads to the formation of a compound film not only on the substrate as desired, but also on the sputtering target. This results in a significantly reduced sputter yield, and thereby, a

Significant pull-out adherence differences were found function of sputtering deposition regime. In the case of BG3-1 and BG3-2 films, the failure occurred in the epoxy adhesive's volume at 84.8 ± 1.5 MPa without damaging the film integrity (G.E. Stan et al., 2011). As this value represents the bonding limit of the epoxy adhesive as confirmed by the manufacturer, the true BG coating – Ti substrate bonding strength could be even higher. This adhesion value is much higher than the usual ones reported in literature (Mardare et al., 2003; Goller,

substrate distance (only 30 mm) (Palmero et al., 2007, G.E. Stan et al., 2010d).

energy of bombarding ions, determining significant variations of the sputter yield.

Fig. 5. SEM images of as-deposited BG3 sputtered films

reduced deposition rate (G.E. Stan et al., 2010a).

2004; Peddi et al., 2008). For the films deposited at higher argon pressure (BG3-3), the adherence dramatically decreased to a mean value of 34.2 ± 12.0 MPa, the bonding failure occurring at the film - substrate interface. When using a reactive atmosphere (BG3-4), but keeping the total pressure constant at 0.3 Pa, the adhesion strength of monolithic BG coating declined down to 44.0 ± 6.8 MPa (G.E. Stan et al., 2011). Similar adherence values have been generally reported in literature (Mardare et al., 2003; Goller, 2004; Peddi et al., 2008). The two tailed t-testing, assuming unequal variances, showed statistically significant differences between the results obtained for the different coatings (p<0.05).

The excellent adherence value of BG3-1 and BG3-2 films must be emphasized. The adherence values are significantly higher than those reported in literature for this kind of implant coatings, thereby opening new perspectives in implantology. The high adherence is related to the processes characteristic of magnetron plasma sputtering. At a lower pressure the sputtered atoms collide with the substrate with higher kinetic energy, creating the possibility of forming chemical bonds with atoms from the substrate or being implanted into substrate. Such phenomena could lead to an increased adherence in case of the BG3-1 and BG3-2 films. Using a higher pressure (BG3-3) results in a spatial density variation of the background gas and affects the magnetron sputtering discharge as well as the transport of the particles towards the substrate. The energy flux at the substrate is thus affected, which in turn affects the properties of the growing film such as density, grain size, columnar structure, stoichiometry, coverage and adhesion (G.E. Stan et al., 2011). Moreover, their surface mobility is dramatically reduced, causing possible film inhomogeneities, such as voids or clustering hillocks (G.E. Stan et al., 2010d). In case of BG3-4 films, the presence of the tower shaped glassy nano-aggregates on the BG surface could be the underlying reason for the decreased adherence. Usually when dealing with tensile forces, rupture involves only a few molecules in the material causing the whole specimen to fracture in a "domino effect" (G.E. Stan et al., 2011).
