**4.1.2.1 Phase analysis**

Three different mixtures of titania (rutile) with the named oxides of trivalent cobalt, nickel and chrome where sprayed on structural steel SJ235R. X-ray diffraction analysis were performed on the coating systems using copper radiation, the diffraction patterns are plotted in **Figure 10** with an offset of 500 counts between the samples. The patterns where checked regarding the presence of unmelted or recrystallized feedstock, the possible solutions as well as reduced oxides and reaction products of the feedstock with the flux melting agent. Because of the marginal coating thickness of some tens of micrometres, the influence of the substrate is recorded in the patterns. As the relative intensity of reflection (RIR) of ferrite is considerably higher than that of the other phases present in the coatings, its peaks are of highest intensity (see the peaks at approximately 45 und 75° 2θ). Since no

structure data is available for the aspired solid solutions, the reference intensity ratio stated in the ICDD PDF4 database entries where used to perform semi quantitative analysis. As no RIRs are given for the solid solutions in the powder diffraction files, values of phases with nearly identical stoichiometry where assumed. The fractions of ferrite were deducted and the adjusted phase contents of the coatings are given in **Table 6**.

Fig. 10. Diffraction patterns of three suspension plasma sprayed coating systems

Three different mixtures of titania (rutile) with the named oxides of trivalent cobalt, nickel and chrome where sprayed on structural steel SJ235R. X-ray diffraction analysis were performed on the coating systems using copper radiation, the diffraction patterns are plotted in **Figure 10** with an offset of 500 counts between the samples. The patterns where checked regarding the presence of unmelted or recrystallized feedstock, the possible solutions as well as reduced oxides and reaction products of the feedstock with the flux melting agent. Because of the marginal coating thickness of some tens of micrometres, the influence of the substrate is recorded in the patterns. As the relative intensity of reflection (RIR) of ferrite is considerably higher than that of the other phases present in the coatings, its peaks are of highest intensity (see the peaks at approximately 45 und 75° 2θ). Since no

structure data is available for the aspired solid solutions, the reference intensity ratio stated in the ICDD PDF4 database entries where used to perform semi quantitative analysis. As no RIRs are given for the solid solutions in the powder diffraction files, values of phases with nearly identical stoichiometry where assumed. The fractions of ferrite were deducted and

the adjusted phase contents of the coatings are given in **Table 6**.

Fig. 10. Diffraction patterns of three suspension plasma sprayed coating systems

**4.1.2 Results** 

**4.1.2.1 Phase analysis** 


Table 6. Phase contents of the three coating systems

In case of the Ni- and Co-containing coatings, significant amounts of Ti(IV)-oxides were measured, of which approximately one third is anatase. As stated in (Bolelli, G., et al., 2009), in case of rutile feedstock the phase content of anatase especially in suspension sprayed coatings can be explained by slow cooling due to re-solidification of molten droplets in the process, compared to formation of rutile in rapid quenching on the substrate. Considering this explanation another assumption might be the influence of elevated substrate temperatures in the SPS process leading to a more slowly cooling of molten titania particles after impinging on the substrate. To distinguish both possible mechanisms further investigations will be conducted considering the thermodynamics of the phase changes of both titania species. In the case that the anatase content correlates well with the content of re-solidificated particles in the coating, the anatase-to-rutile ratio can be used to optimize the injection and spraying parameters.

For the coatings containing nickel, about 7% percent of Ni(II)-oxide were found, whereas in titania-cobalt-oxide systems no remains of the Co-feedstock was detected. The employed trivalent oxides of both cations decompose towards the divalent oxide at temperatures above approximately 600° in case of the Ni-oxide and 1910° C for the Co2O3. Otherwise the contents of borates formed by reactions of the boron oxide with the feedstock oxides is three times higher for the Co-based system compared to the titania-nickel-oxide coating. As the absolute value of the enthalpy of formation of the cobalt-borate is higher than that of the Niborate (Hawk, D. and Müller, F.; 1980; Paul, A., 1975), the Co-oxide feedstock is diluted in the boron oxide to a much higher extent compared to the Ni-containing system, and no remaining Co2O3 is embedded in the coating. In contrary to that the contents of Ni-borates are small in the titania-Ni-oxide coating, and remains of the Ni(II)-oxide are recorded. The phase contents of the aspired solid solutions are below 30 % for both coatings systems.

Compared to the Ni- and Co-containing coatings the mixing of titania with chromia leeds to different phase compositions. Due to the marginal miscibility of chromia with boron oxide (Tombs, N. C.; Croft, W. J. & Mattraw, H. C.; 1963), no borates and also just small amounts of the feedstock powders are found. The Andersson-phases with the mentioned stoichiometry of Tin-2Cr2O2n-1 amount to three quarters of the total coatings composition. Therefore it can be concluded, that the degree of mixing of the feedstock is significantly higher for the titania-chromia system. If the melted phase of the boron oxide supports the mixture process of the both oxide ceramics without further reaction cannot be clarified. Possibly the heat of the process is better transferred to the coarser feedstock of approximatly 100 nm median crystallite size compared to 30 to 60 nm of the feedstock of the Ni- and Cocontaining coatings. As the heat transfer degreases drastically when the agglomerate size of the feedstock particles falls below a critical limit (so called Knudsen effect, Fauchais, P. et al., 2008), this might be a supposable explanation of the higher degree of feedstock mixing in the case of the titania-chromia system.

Thermal Spraying of Oxide Ceramic and Ceramic Metallic Coatings 185

the ball (see the debris of the ball on the coating in the second picture from top on the left hand in **Figure 12**), but this effect is desired as the billet in the extrusion process shows a comparable behavior. For this reason, the testing of the coatings in tribometer experiments is not directly comparable to the hot extrusion process, as the soft consistency of the flowing billet above yield stress cannot be tested because the ball would be abraded promptly and its holder would scratch the coating. But compared to the given values of operating unlubricated containers of more than 0.2 (Bauser, M.; Sauer, G. and Siegert, K., 2006), a significant lower frictional force was measured. Besides the tribological activity of the coating it shows good material separating properties against 100Cr6. When rising the temperatures to 800° C, the COF rises again to values of nearly 0.2. An explanation is the formation of black ferrous oxide (supposable magnetite) instead of the red oxide (presumably haematite), showing higher hardness and unfavourable tribolological

Fig. 12. Top views of the scare tracks left and corresponding friction surfaces of the

counterparts from room temperature (top) to 800° C (bottom)

properties (Barbezat, G., 2006).

In addition, with approximately 10 % significant amounts of chromium are present in the coatings, being formed by reduction of the chromia feedstock. This effect is only detected when spraying the suspension with the Triplex-II and not when using the DELTA-Gun, and further on when besides chromia titania is present in the suspension. This result is probably due to the large gap between the absolute values of the Gibbs free energy of the two oxides. Hence the chromia is reduced in the presence of titania. By means of visible spectroscopy protons where found supposedly originating from the vaporization of the water of the suspension, but no ions of oxygen where detected. Together with the lamellar flow of the plasma jet of the Triplex resulting in marginal entrainment of surrounding air, apparently the conditions are given for the reduction of the chromia towards chrome.

### **4.1.2.2 Tribological testing of Andersson type coatings**

Since the contents of the solid solutions of titania with another oxide were the highest in case for the titania-chromia system, tribological testing for recording the coefficient of friction dependent on the temperature of operation were conducted with coatings of this Andersson type phases using a ball on disk configuration. The coatings rotated against a ball of 100Cr6 (1.3505, diameter = 5 mm) with 0.1 m/s, the loading force was 5 N. The coefficient of friction was recorded in three runs on different samples at room temperature, 600° and 800° C (see **Figure 11**).

Fig. 11. COF of Andersson type coating systems measured against 100Cr6 at RT, 600° and 800° C

The friction pairing shows a COF of more than 0.6 when running at room temperature. When rising the temperatures up to 600° C, the ratio of the frictional force to loading force drops considerably to below 0.1. On the one hand this effect is surely due to the softening of

In addition, with approximately 10 % significant amounts of chromium are present in the coatings, being formed by reduction of the chromia feedstock. This effect is only detected when spraying the suspension with the Triplex-II and not when using the DELTA-Gun, and further on when besides chromia titania is present in the suspension. This result is probably due to the large gap between the absolute values of the Gibbs free energy of the two oxides. Hence the chromia is reduced in the presence of titania. By means of visible spectroscopy protons where found supposedly originating from the vaporization of the water of the suspension, but no ions of oxygen where detected. Together with the lamellar flow of the plasma jet of the Triplex resulting in marginal entrainment of surrounding air, apparently

Since the contents of the solid solutions of titania with another oxide were the highest in case for the titania-chromia system, tribological testing for recording the coefficient of friction dependent on the temperature of operation were conducted with coatings of this Andersson type phases using a ball on disk configuration. The coatings rotated against a ball of 100Cr6 (1.3505, diameter = 5 mm) with 0.1 m/s, the loading force was 5 N. The coefficient of friction was recorded in three runs on different samples at room temperature,

Fig. 11. COF of Andersson type coating systems measured against 100Cr6 at RT, 600° and

The friction pairing shows a COF of more than 0.6 when running at room temperature. When rising the temperatures up to 600° C, the ratio of the frictional force to loading force drops considerably to below 0.1. On the one hand this effect is surely due to the softening of

the conditions are given for the reduction of the chromia towards chrome.

**4.1.2.2 Tribological testing of Andersson type coatings** 

600° and 800° C (see **Figure 11**).

800° C

the ball (see the debris of the ball on the coating in the second picture from top on the left hand in **Figure 12**), but this effect is desired as the billet in the extrusion process shows a comparable behavior. For this reason, the testing of the coatings in tribometer experiments is not directly comparable to the hot extrusion process, as the soft consistency of the flowing billet above yield stress cannot be tested because the ball would be abraded promptly and its holder would scratch the coating. But compared to the given values of operating unlubricated containers of more than 0.2 (Bauser, M.; Sauer, G. and Siegert, K., 2006), a significant lower frictional force was measured. Besides the tribological activity of the coating it shows good material separating properties against 100Cr6. When rising the temperatures to 800° C, the COF rises again to values of nearly 0.2. An explanation is the formation of black ferrous oxide (supposable magnetite) instead of the red oxide (presumably haematite), showing higher hardness and unfavourable tribolological properties (Barbezat, G., 2006).

Fig. 12. Top views of the scare tracks left and corresponding friction surfaces of the counterparts from room temperature (top) to 800° C (bottom)

Thermal Spraying of Oxide Ceramic and Ceramic Metallic Coatings 187

makes use of three anodes to combine high power inputs into the plasma as well as stable process conditions. Besides a more narrow nozzle outlet diameter compared to multicathode designs hydrogen can be used as secondary plasma gas, both resulting in higher plasma velocities and net powers. The conceptional designs of two guns are discussed as well as their suitability for suspension and shrouded plasma spraying. The efforts in

To overcome the disadvantages of conventional plasma guns especialy regarding the discontinuity of the free jet due to plasma arc root rotation, mulitelectrode guns were developed. Since more than ten years guns basing on the three-cathode-design guarantee high plasma net powers combined with stable feedstock injection conditions. Until now the guns have two disadvantages concerning the use of expensive helium as secondary gas accompanied by low plasma arc voltages on the one hand and the restriction of the minimal nozzle outlet diameter on the other. For example three single plasma fingers originate from the single cathodes being passed through a cascaded neutrode in case of the second generation of the Triplex-design (Sulzer Metco AG, Wohlen/Switzerland). Hence a minimal nozzle outlet diameter of the anode of 9 mm can be realized because of the thermal design of the gun. Another approach is the inverted design of a plasmatron, where one arc originates from a single cathode and is divided on three anodes after passing the cascade. Therefore for the DELTA-Gun (GTV GmbH, Luckenbach/Germany) a minimal nozzle outlet diameter of 7 mm can be achieved resulting in higher plasma velocities at the nozzle outlet. Furthermore hydrogen can be used as secondary gas and high brut plasma powers of 80 kW can be applied to the torch.

The workings concentrated on the investigations, to what extent both gun concepts are appropriate for inert and reactive shrouded plasma spraying as well as the processing of nanoscaled suspensions. Feedstock was used being not commonly applied in plasma spraying to identify the potential of plasma spraying for possibly new applications. For demonstration purposes coating systems of titanium and chromium as well as their nitrides and Indium-Tin-Oxide (ITO) showing electrical conductance were chosen. For the chromium coatings feedstock obtained from GTV GmbH with two different particle size distributions (-25+5 µm and -45+5 µm) were investigated. As titanium feedstock a powder of -45+10 µm came into operation, which is manufactured and distributed by TLS Technik Spezialpulver GmbH (Bitterfeld/Germany). For SPS suspensions containing 5 wt.-% ITO (ANM PH 15695, Evonik Degussa GmbH, Marl/Germany) and Al2O3 (Saint Gobain, Weilerswist/Germany) with primary crystallite sizes of some tens for the first and

For both guns modules have been designed and machined to apply shroud gases around the plasma free jet (for details see **Figure 14** on the following page). The attachments consist of water-cooled bodies, in which the shrouding gas is injected helically to ensure a sufficient

achieving new plasma sprayed coating systems are presented.

**4.2.1 Design of marketable multielectrode plasma guns** 

**4.2.2 Experimental** 

**4.2.3 Results** 

approximately 150 nm for the latter were used.

**4.2.3.1 Shrouded plasma spraying** 

This finding is another example for the low comparability between tribometer experiment and hot extrusion, as the billet is pressed under air exclusion in the container. Another guess is a lack in thermal stability of the Andersson phases. Other SPS coatings sprayed with the same feedstock composition where tempered at different temperatures (300, 500 and 800° C) for several hours. The colouring of the coatings changed with temperature (see top views on the left hand side of **Figure 13**), as the phase composition changes (see the corresponding diffraction patterns on the right). The marked peaks in the diffraction pattern of the sample tempered at 500° C are caused by the Andersson phases. As clearly can be seen, this peaks are significantly smaller in the sample being not tempered and that one tempered at 800° C. So it can be stated, that the tribological active phases can be formed with rising temperature within the coatings during operation, but also may decompose with further increased temperatures.

Fig. 13. Alteration of the phase composition of Andersson type coatings in relation to temperature (right side) and corresponding top views of the tempered samples
