**2. Preparation and characterization of deposits**

In order to carry out our own research on the possibility of obtaining ultra-hard coatings based on WC and TiC by thermal spraying, we made a spraying head system, which we have adapted to a classic drive mechanism, [32]. The schematic overview of the spraying device is presented in **Figure 4**.

The designed spray device consists of two distinct modules:


The wire feed mechanism - realized by IOR Bucharest, Romania, consists of a direct current electric motor, two-step speed reducer (vertical stage) and electronic control system. It develops at the output a power Pie of 0.063 kW and a speed nmax of 10 rpm. The overview of the wire feed mechanism is shown in **Figure 5**.

The spray module (usually called "spray head") is designed from a system of concentric nozzles - see **Figure 6**, convergent at the electric arc level, capable to ensure a convergent – divergent geometry, of the compressed air jet.

The components elements of the nozzle system are made of insulating materials such as: textolite, high density polypropylene and polyurethane of Moldotan type.

The concentric nozzle system, together with the module body, forms two compressed air circuits [32, 33], as presented in **Figure 7**:


The compressed air which passes through the *main circuit* determines the detachment of the liquid droplets from the wires surface and the dispersion of the droplet formed into fine particles. This circuit influence thus the size and the speed of the sprayed particles, [32]. The compressed air that passes through the secondary circuit has the role of constraining the electric arc, determining the increase of the current density and implicitly of the particle temperature. It is directed by the inner surface of the constraint frontal nozzle - see **Figure 8**, to the forming area of the electric arc. The two compressed air circuits are supplied from two different sources, with different pressures.

**143**

**Figure 9**, [11].

**Figure 5.**

**Figure 6.**

*Wire feed mechanism assembly.*

**2.1 Technological workflow**

*insulator; 5-wire guides; 6-thread; 7-module body.*

additional material is presented in **Table 1**.

In our research substrates made of low alloy steel C15 –EN10083, with dimensions of 40 mm x 40 mm x 10 mm were covered by arc spraying process using as additional material 97MXC - in the form of cored wires, product of the company Praxair-Tafa, USA. The chemical composition of the substrate, as well as that of the

*Spray head: Isometric view - exploded:1- front cover; 2-front nozzle; 3-conical nozzle with grooves; 4- conical* 

In order to produce the ultra-hard coatings with WC and TiC content, we used an electric arc spraying installation, provided with a compressed air compressor, which ensures pressures of 8 bar and a flow of up to 700mc/min, a direct current

The substrate surface activation included a series of preparatory operations for the metallization stage, which aimed both to clean the surface from oxides, oils and

source RSC 400 type and the spray device presented in **Figure 4**.

The stages of the technological flow are schematically presented in

*Hard Alloys with High Content of WC and TiC—Deposited by Arc Spraying Process*

*DOI: http://dx.doi.org/10.5772/intechopen.94605*

**Figure 4.** *Spray device - schematic overview.*

*Hard Alloys with High Content of WC and TiC—Deposited by Arc Spraying Process DOI: http://dx.doi.org/10.5772/intechopen.94605*

**Figure 5.** *Wire feed mechanism assembly.*

*Welding - Modern Topics*

• wire feed module;

• spraying module.

**2. Preparation and characterization of deposits**

overview of the spraying device is presented in **Figure 4**.

The designed spray device consists of two distinct modules:

ensure a convergent – divergent geometry, of the compressed air jet.

the conical nozzle with grooves and the body of the module.

compressed air circuits [32, 33], as presented in **Figure 7**:

nozzle with grooves and the conical insulator;

sources, with different pressures.

In order to carry out our own research on the possibility of obtaining ultra-hard

The wire feed mechanism - realized by IOR Bucharest, Romania, consists of a direct current electric motor, two-step speed reducer (vertical stage) and electronic control system. It develops at the output a power Pie of 0.063 kW and a speed nmax of 10 rpm. The overview of the wire feed mechanism is shown in **Figure 5**.

The spray module (usually called "spray head") is designed from a system of concentric nozzles - see **Figure 6**, convergent at the electric arc level, capable to

The components elements of the nozzle system are made of insulating materials such as: textolite, high density polypropylene and polyurethane of Moldotan type. The concentric nozzle system, together with the module body, forms two

• the main circuit - formed in the space between the module body, the conical

The compressed air which passes through the *main circuit* determines the detachment of the liquid droplets from the wires surface and the dispersion of the droplet formed into fine particles. This circuit influence thus the size and the speed of the sprayed particles, [32]. The compressed air that passes through the secondary circuit has the role of constraining the electric arc, determining the increase of the current density and implicitly of the particle temperature. It is directed by the inner surface of the constraint frontal nozzle - see **Figure 8**, to the forming area of the electric arc. The two compressed air circuits are supplied from two different

• the secondary circuit - formed in the space between the cover, the front nozzle,

coatings based on WC and TiC by thermal spraying, we made a spraying head system, which we have adapted to a classic drive mechanism, [32]. The schematic

**142**

**Figure 4.**

*Spray device - schematic overview.*

#### **Figure 6.**

*Spray head: Isometric view - exploded:1- front cover; 2-front nozzle; 3-conical nozzle with grooves; 4- conical insulator; 5-wire guides; 6-thread; 7-module body.*

#### **2.1 Technological workflow**

In our research substrates made of low alloy steel C15 –EN10083, with dimensions of 40 mm x 40 mm x 10 mm were covered by arc spraying process using as additional material 97MXC - in the form of cored wires, product of the company Praxair-Tafa, USA. The chemical composition of the substrate, as well as that of the additional material is presented in **Table 1**.

In order to produce the ultra-hard coatings with WC and TiC content, we used an electric arc spraying installation, provided with a compressed air compressor, which ensures pressures of 8 bar and a flow of up to 700mc/min, a direct current source RSC 400 type and the spray device presented in **Figure 4**.

The stages of the technological flow are schematically presented in **Figure 9**, [11].

The substrate surface activation included a series of preparatory operations for the metallization stage, which aimed both to clean the surface from oxides, oils and

**Figure 7.** *Operating diagram of the spray head, [32].*

**Figure 8.**

*The frontal constraint nozzle, [32].*


#### **Table 1.**

*Chemical composition of the materials.*

greases, and to increase its roughness, in order to ensure good adhesion of the coating to the substrate, [32, 34]. The surface cleaning operation was performed in two successive phases: chemical cleaning and mechanical cleaning.

Chemical cleaning consisted of several steps: washing the substrate surface in a jet of liquid solution with a concentration of 10% (5% caustic soda, 5% soda salt), at a temperature of 50°C; rinsing in a stream of hot water at a temperature of 90°C; degreasing in methylene tetrachloride; wiping the substrate surface until dry with a textile material, [31].

**145**

electric arc thermal spray process.

**2.2 The characterization of ultra-hard 97MXC coatings**

of the WC/TiC ultra-hard coatings obtained by arc spraying process:

*Hard Alloys with High Content of WC and TiC—Deposited by Arc Spraying Process*

**Parameters Value** Current intensity (A) 200/220/250 Voltage (U) 32 Air pressure in the primary circuit (bar) 5.5/6.0/6.5 Air pressure in the secondary circuit (bar) 3.0 Movement speed of the gun (m/s) 0.14 Coating thickness (mm) 0.5–0.7 Stand-off distance (SOD) (mm) 110

The subsequent phase, respectively the mechanical cleaning, consisted in sandblasting the surface of the substrate with abrasive particles. This process required pressurizing the abrasive medium with the help of compressed air and directing the flow of abrasive particles on the surface of the substrate [35]. Hard, abrasive and sharp particles of aluminum oxide with an average diameter of 536 ± 124 μm, sprayed with a pressure of 4barr at a distance of 30 mm, were used for blasting. The surface roughness obtained was between 46 and 62 μm. However, sandblasting always carries great risk, respectively it leaves the surface of the substrate contaminated with abrasive particles entrapped in the material, [36–38]. These residues have negative effects on the mechanical properties of the coatings, such as: they reduce the adhesion of the layer to the substrate, reduce the fatigue resistance properties of the substrate, limits the diffusion between coating and substrate and reduces the contact surface between particles [39–41]. To prevent the mentioned aspects, the substrate surface was ultrasonically cleaned by immersing the specimens in an ethanol bath (C2H6O) for 10 minutes, followed by drying them under a jet of filtered compressed air.

In our research, two process parameters, respectively: the compressed air pressure passing on the primary circuit and the intensity of the electric current, varied on three levels. For a good analysis of the effect of these variations, the rest of the technological parameters were kept constant. **Table 2** shows the parameters of the

This subchapter presents the following investigations performed for the analysis

*DOI: http://dx.doi.org/10.5772/intechopen.94605*

**Figure 9.**

**Table 2.**

*Thermal spray parameters.*

*Technological workflow.*

*Hard Alloys with High Content of WC and TiC—Deposited by Arc Spraying Process DOI: http://dx.doi.org/10.5772/intechopen.94605*

#### **Figure 9.** *Technological workflow.*

*Welding - Modern Topics*

**144**

**Table 1.**

**Figure 8.**

**Figure 7.**

*Operating diagram of the spray head, [32].*

textile material, [31].

*Chemical composition of the materials.*

*The frontal constraint nozzle, [32].*

greases, and to increase its roughness, in order to ensure good adhesion of the coating to the substrate, [32, 34]. The surface cleaning operation was performed in two

C15 0,14 0,15 0,3 0,3 0,43 — — — — 0,04 0,04 97MXC — 1,25 14,0 4,5 0,55 1,87 26,0 6,0 balance — —

**C Si Cr Ni Mn B WC TiC Fe P S**

Chemical cleaning consisted of several steps: washing the substrate surface in a jet of liquid solution with a concentration of 10% (5% caustic soda, 5% soda salt), at a temperature of 50°C; rinsing in a stream of hot water at a temperature of 90°C; degreasing in methylene tetrachloride; wiping the substrate surface until dry with a

successive phases: chemical cleaning and mechanical cleaning.

**Materials Elements (wt%)**


#### **Table 2.**

*Thermal spray parameters.*

The subsequent phase, respectively the mechanical cleaning, consisted in sandblasting the surface of the substrate with abrasive particles. This process required pressurizing the abrasive medium with the help of compressed air and directing the flow of abrasive particles on the surface of the substrate [35]. Hard, abrasive and sharp particles of aluminum oxide with an average diameter of 536 ± 124 μm, sprayed with a pressure of 4barr at a distance of 30 mm, were used for blasting. The surface roughness obtained was between 46 and 62 μm. However, sandblasting always carries great risk, respectively it leaves the surface of the substrate contaminated with abrasive particles entrapped in the material, [36–38]. These residues have negative effects on the mechanical properties of the coatings, such as: they reduce the adhesion of the layer to the substrate, reduce the fatigue resistance properties of the substrate, limits the diffusion between coating and substrate and reduces the contact surface between particles [39–41]. To prevent the mentioned aspects, the substrate surface was ultrasonically cleaned by immersing the specimens in an ethanol bath (C2H6O) for 10 minutes, followed by drying them under a jet of filtered compressed air.

In our research, two process parameters, respectively: the compressed air pressure passing on the primary circuit and the intensity of the electric current, varied on three levels. For a good analysis of the effect of these variations, the rest of the technological parameters were kept constant. **Table 2** shows the parameters of the electric arc thermal spray process.

#### **2.2 The characterization of ultra-hard 97MXC coatings**

This subchapter presents the following investigations performed for the analysis of the WC/TiC ultra-hard coatings obtained by arc spraying process:

