Suspension Plasma Spraying (SPS) of Cobalt Spinel

Fine (median size 6 μm and 0.3 μm) cobalt spinel (Co3O4) powders were processed suspended in a suitable liquid phase. Suspensions exceeding 50 wt.% solid phase content were successfully injected into an inductively coupled plasma. Spheroidized powders with large particle size (up to 80 μm) were prepared, and cobalt oxide coatings were produced by this novel RF-SPS method. The microstructural features of the coatings can be controlled by parameter optimization similarly to plasma spraying of dry powders. Numerous variations of the physical and chemical conditions of the process were performed in an attempt to overcome the main disadvantage of the process, i.e. the decomposition of the spinel phase to CoO. So far, the spinel phase could be reestablished only by a post-treatment of the deposited coatings with atomic oxygen in the RF plasma.

Characterization of Cr2O3-TiO2-CaF2 Coatings Using Plasma Spray Process

This paper presents the results of a study on the wear resistance of plasma spray coatings made from Cr2O3-TiO2-CaF2 powders. The composite powders used were produced by self-propagating high temperature synthesis. They were then applied under various conditions in order to optimize the material system, spray process, and application procedures. Based on the results of microstructural examination and wear testing, the thermally sprayed composite coatings have excellent wear resistance, good adhesion, and are self-lubricating at high temperatures.

Slurry Erosion of High Velocity Oxy-fuel Thermal Sprayed Coatings

Improvement of the high velocity oxy-fuel deposition (HVOF) process in the last decade has enhanced the microstructure of coatings in order to better perform against wear and corrosion. Indeed cermet and metal HVOF coatings are reliable and have excellent performance under slurry erosion and provide therefore an alternative to the use of high-priced material. This paper presents the results of a study undertaken within the core research program of the National Research Council of Canada technology group in surface engineering, "SURFTEC", in which the performance of ten HVOF erosion-resistant coatings was evaluated. Ten different types of HVOF coatings were studied including: six grades of WC with either Co or a Ni based matrix, one grade of Cr3C2 in a Ni-Cr matrix, and three grade of metallic alloy: Ni alloy, Co alloy and a SS 316- L. The performance of coatings was evaluated with respect to: the volume ratio and composition of metallic binder in carbide coatings, type of carbide, coating microstructure, impinging angle and the size of the erodent particles. All coatings were produced using the HVOF JP-5000 system controlled by the Hawcs-ll controller. Slurry erosion tests were conducted with a jet impingement rig with a 10 %w/w alumina particle/water slurry. The volume loss of material under various slurry erosion conditions was related to the coating properties and microstructure. Results indicate that the behavior of HVOF sprayed materials is dependent on the erodent particle size, to the erosion impinging angle to some extent and to the corrosion resistance of the cermet matrix.

Influence of Surface Condition on the Bonding Strength of Thermal Sprayed Coatings

The role of surface roughness in coating adhesion mechanism is studying for detonation spraying. Roughness was produced by conventional grit blasting, D-gun blasting and was formed as a result of spraying of high-adhesive thin layer of detonation coating. Cermet and alloy powders were sprayed by detonation gun Ob. The coating bonding strength measurements show the WC+25Co adhesion to be above 200 MPa independently of a substrate surface preparation. Contrary, NiCrSiB coatings are very sensitive to surface conditions their adhesion varies from 180 MPa to zero. As-sprayed alloy particles fail in adherence because of insufficient energy to fuse substrate material at a flat surface. Only developed (wide scale) roughness may be fused partially by these particles for their bonding to the substrate. Otherwise, high heated cermet particles do not need special surface preparation (except cleaning) for fusion of substrate material to provide high bonding with it. The wide scale and ball shape roughness, which is similar to the self-reproduced coating roughness, provides the best conditions for the coating bonding and it is recommended as the purpose of surface treatment before thermal spray coating.

Fracture Behavior and Tensile Adhesive Properties of HA Coatings

The attractive bioactive properties of HA are significantly reduced upon plasma spraying because of the phase transformation that accompanied the deposition process. One major factor that influence the extent to which the transformation occur appears to be the morphology and physical states of the HA raw powders. This paper reports the study on the influence of powder morphology and property on the fracture behaviour and tensile adhesive strength of plasma sprayed HA coatings. Three types of powders were used in the study; calcined HA (CHA), spray dried HA (SDHA) and flame spheroidised HA (SHA). The particle size range of 53 - 75 μm was employed for all 3 types of powders to effect an accurate comparison of the powders. Results show that the cohesive bond strength of the SHA coating was the highest because of the denser microstructure created by well-formed lamella splats. A correspondingly lower bond strength was recorded with less coherent coatings generated by agglomerated CHA and SDHA powders.

Influence of WC Particle Size and Matrix Composition on the Behavior of WC-Co-Cr Coatings Sprayed by the HVOF Process

WC-Co-Cr powders with different WC particle size have been sprayed by the HVOF process. At constant spraying conditions the powders give coatings of different quality. The deposition efficiency during spraying of powders containing large WC particles was found to be low compared to powders with finer WC grains. In addition the amounts of porosity and cracks were different. The coatings have been characterised by different methods. Erosion and erosion-corrosion tests showed that the WC particle size also influence the wear resistance of the coatings. Small WC particle size was found to be beneficial. Chemical composition of the matrix was also found to be decisive for the coating properties. An increase of the chromium content improved the erosion-corrosion resistance.

Measurement of the Crystallinity of Hydroxyapatite Deposited by Plasma Spray

As part of a characterization and mechanical research about hydroxyapatite (HA) plasma deposits for hip prosthesis, we addressed the problem of determining their crystallinity. A traditional normalization method employed by several laboratories is based on X-ray diffraction by a powder mixture of the investigated HA sample with a standard of crystalline powder, namely Al2O3. This method is quite unsatisfactory, as very often delivers unreasonable results. In order to overcome these difficulties we investigated some new methods for determining the crystallinity of HA sample, which are based on X-ray diffraction. All these methods provide reasonable results.

High Velocity Oxy-Fuel Thermal Sprayed Coatings as Alternates to WC-12Co Coatings and Chromium Plating

Tungsten carbide-12 wt.% cobalt (WC-12Co) coatings and chromium plating are used to provide wear resistant surfaces in gas turbine applications. These treatments provide surfaces with hardnesses greater than 60 Rockwell C. In addition, a surface finish better than 8 microinches RMS is required for optimum performance. To achieve this surface finish, diamond grinding is required. The diamond grinding step adds considerable cost to the product and economical benefits could be achieved if more conventional grinding techniques were incorporated. A program was initiated to develop an alternative thermal spray coating, with a target hardness lower than 60 Rockwell C, but high enough to provide the wear resistance required. Spray development was conducted on five commercially available materials using the Diamond Jet 2600 high velocity oxy-fuel process. Laboratory evaluation included coating microstructure, macro- and microhardness, bond strength, salt spray corrosion, and cyclic compression tests.

Microstructure and Abrasion Resistance of WC-Co Coatings Produced by High Velocity Oxy-Fuel Spraying

Coatings have been produced by HVOF spraying of four different WC-Co powders, using two fuel gases and two oxygen contents in the flame, and characterised in terms of microstructure and resistance to abrasive wear. It is concluded that there is a close correlation between high levels of chemical reaction, occurring during spraying (and possibly during powder production), and poor wear resistance. Good wear resistance is favoured by using low porosity powders, which interact with the atmosphere less readily during spraying, and also by using a flame with a relatively low oxygen content. This probably minimises the degree of reaction by ensuring that conditions are reducing. Use of propylene rather than hydrogen gives coatings with slightly better wear resistance, despite the fact that the flame temperatures are higher. It is concluded that, for this relatively small rise in temperature, the positive effect on inter-splat cohesion seems to outweigh the negative effect of increased decarburisation.

HVOF- and VPS-Coatings Using Nanostructured Iron-Based Alloys

In thermal spraying, Fe-based alloys are often applied for relatively thick and inexpensive coatings. The main advantage of the Fe-based alloy coatings is their high ductility as compared to ceramic and hardmetal coatings. Other advantages such as high toughness, easy machineability and satisfactory corrosion resistance are characteristic of Fe-based alloys. The wear resistance is not outstanding, but nevertheless acceptable for a large number of applications. A further improvement of the wear resistance can be achieved by reinforcing the Fe-based alloy coatings, e.g. by addition of nitrogen to the spraying powder.