P.H. Shipway

Abrasive wear behaviour of conventional and nanocomposite HVOF-sprayed WC–Co coatings

Abstract WC–Co cermets have been used traditionally as wear-resistant materials. Recent work has shown that nanostructured cermets offer improved properties over their conventional counterparts. This work examines the performance of such conventional and nanostructured materials in the form of coatings deposited by high velocity oxy-fuel (HVOF) thermal spraying. WC–Co coatings were deposited under identical conditions using both conventional sintered and crushed and nanocomposite powder feedstocks. Both powders consisted of tungsten carbide (WC) grains in a cobalt binder. Characterisation of the coatings by a range of techniques showed that both coatings not only contained WC but also reaction products such as tungsten hemicarbide (W 2 C) and W and an amorphous Co-rich binder phase containing W and C. Due to differences in the morphology of the powder feedstock and the WC grain size, the nanocomposite coating contained a smaller fraction of unreacted WC than the conventional coating. Three body abrasive wear tests were performed using a modified dry sand rubber wheel apparatus with alumina and silica abrasives. A range of abrasive particle sizes and loads were used to assess the wear resistance of both coatings. It was found that the nanocomposite had a poorer wear resistance than the conventional coating under all the conditions examined. Wear was dominated by the loss of ductility in the Co-rich binder phase due to its amorphisation. The differences in the wear behaviour of the coatings could, thus, be explained in terms of differences in powder characteristics, the extent of reaction and decarburisation during spraying, and the subsequent development of the microstructure in the coating during splat solidification at high cooling rates.

The rôle of particle properties in the erosion of brittle materials

The erosion of a range of brittle materials (soda-lime glass, borosilicate glass, fused silica, boron carbide, partially stabilized zirconia, alumina and silicon carbide) with number of different erodent particle types (silicon, silica, alumina and silicon carbide) has been examined. The mechanisms of erosion depend upon the ratio of particle to target hardness. As this ratio falls towards unity, less damaging mechanisms of erosion dominate. Indentation-induced fracture models for erosion of brittle materials are not valid when this mechanism ceases to operate. The erosion rates decrease rapidly, and the velocity exponents of erosion rate increase, as the ratio of particle to target hardness decreases towards unity. The consequences of this in accelerated erosion testing are addressed.

Fracture of brittle spheres under compression and impact loading. I. Elastic stress distributions

Abstract Numerical values are derived for the elastic stress fields in spheres under conditions of quasi-static compression and free impact against plane targets. The results are relevant to the brittle fracture of spheres under compression and impact loading and allow some anomalies observed in indirect methods of tensile testing to be clarified.

Production of titanium deposits by cold-gas dynamic spray: Numerical modeling and experimental characterization

Over the past five years, interest in cold-gas dynamics spraying (CGDS) has increased substantially. Considerable effort has been devoted to process development and optimization for such metals as copper and aluminium. This paper describes work undertaken to expand the understanding of the deposition of titanium by cold-spray methods. CGDS deposits have been produced from commercially pure titanium using room-temperature helium gas. The effect of different powder paticle size ranges, types of substrate, substrate preparation methods, and spray parameter conditions on powder deposition have been investigated. Microhardness testing of deposits was conducted, and their microstructures have been examined by scanning electron microscopy. Samples for pull-off bond-strength tests have been prepared from a number of the more promising sets of spray parameters and adhesive strengths determined. A one-dimensional numerical model of particle acceleration, employing isentropic gas flow behavior in the nozzle, has also been used to estimate particle exit velocities. This model explicitly addresses the dependence of the drag coefficient on gas compressibility and demonstrates its significance in terms of predicted particel velocities. By linking this model with the measured particle size distributions, estimates of particle velocity distributions at the nozzle exit plane have been computed. These allow an approximate value of the critical velocity for deposition of titanium to be made. Experimental observations on the microstructure and properties of the deposits are discussed in light of powder particle size and velocity distributions and the underlying physical and mechanical properties of the powders and substrates.

Cold gas dynamic spraying of aluminum: The role of substrate characteristics in deposit formation

Aluminum powder of 99.7 wt.% purity and in the nominal particle size range of −75+15 µm has been sprayed onto a range of substrates by cold gas dynamic spraying (cold spraying) with helium, at room temperature, as the accelerating gas. The substrates examined include metals with a range of hardness, polymers, and ceramics. The substrate surfaces had low roughness (Ra < 0.1 µm) before deposition of aluminum in an attempt to separate effects of mechanical bonding from other forms of bonding, such as chemical or metallurgical bonding. The cross-sectional area of a single track of aluminum sprayed onto the substrate was taken as a measure of the ease of initiation of deposition, assuming that once a coating had begun to deposit onto a substrate, its growth would occur at a constant rate regardless of substrate type. It has been shown that initiation of deposition depends critically upon substrate type. For metals where initiation was not easy, small aluminum particles were deposited preferentially to large ones (due to their higher impact velocities); these may have acted as an interlayer to promote further building of the coating. A number of phenomena have been observed following spraying onto various substrates, such as substrate melting, substrate and particle deformation, and evidence for the formation of a metal-jet (akin to that seen in explosive welding). Such phenomena have been related to the processes occurring during impact of the particles on the substrate. Generally, initiation of aluminum deposition was poor for nonmetallic materials (where no metallic bonding between the particle and substrate was possible) and for very soft metals (in the case of tin, melting of the substrate was observed). Metallic substrates harder than the aluminum particles generally promoted deposition, although deposition onto aluminum alloy was difficult due to the presence of a tenacious oxide layer. Initiation was seen to be rapid on hard metallic substrates, even when deformation of the substrate was not visible.

Attrition of brittle spheres by fracture under compression and impact loading

Abstract Results are presented of a theoretical and experimental study of the fracture of single brittle spheres by uniaxial compression between opposed platens and by free impact against plane targets. The stress distributions in elastic spheres are broadly similar under both types of loading, with significant tensile components inside the sphere on the axis of the system and on the surface of the sphere, around the equator for the case of compression. The magnitudes and locations of the peak values of these stresses depend on the size of the contact area (relative to the size of the sphere), which in turn depends on the mechanical properties of the target and platen material. A simple equation used by previous investigators to estimate the maximum internal tensile from the load provides a useful approximation only under certain conditions, and more generally leads to significant error. Experiments with lead glass spheres have shown that under many conditions of compression and impact testing, failure initiates on the surface, rather than internally, at a critical value of tensile stress. The work is relevant to the breakage attrition of brittle particles in powder transport, handling and processing, and also to comminution. It also provides a basis for the interpretation of indirect tensile tests in which spherical samples are uniaxially compressed.

A method for optimizing the particle flux in erosion testing with a gas-blast apparatus

Abstract The erosive wear rate of a material depends on the flux of particles striking the surface, and in order to measure an erosion rate of a material in which each particle strikes the surface independently of all other particles, as is generally assumed, experiments should be carried out at low particle fluxes. However, the flux used in testing is often chosen arbitrarily with little rational basis for the choice. The mechanisms have been studied by which the erosive conditions depend on particle flux in a gas-blast erosion rig. The occurrence of particle-particle interactions in the region between the nozzle and target, mainly between incoming and rebounding particles, depends strongly on the flux. A novel method has been used to determine the extent of particle-particle interaction, and to determine the flux below which the effects of particle-particle interactions are insignificant. The method facilitates the generation of accurate and meaningful erosion data, within the shortest practicable time.

The influence of HVOF powder feedstock characteristics on the abrasive wear behaviour of CrxCy–NiCr coatings

Abstract Cr x C y –NiCr cermet coatings are commonly utilised in industry as a means to provide wear resistance in high temperature environments. Such coatings are usually deposited by thermal spraying. In this work, Cr x C y –NiCr coatings have been deposited using a Miller Thermal/UTP Top Gun high velocity oxy-fuel spray system from three commercially available powders, namely a sintered and crushed powder, a blended powder and a composite powder. The characteristics of the powder feedstock have a strong influence on the coating microstructures, although the coating microstructure is often very different to that of the powder due to processes that occur during spraying and rapid solidification of the splats. Coating microstructures were assessed by X-ray diffraction, scanning electron microscopy and microhardness measurements. The abrasive wear performance of the coatings was assessed using a dry sand–rubber wheel test with both alumina and silica abradents. It was found that, under all conditions, the coating deposited from the blended powder exhibited the highest wear rates and the coating deposited from the composite powder exhibited the lowest wear rates. Wear mechanisms were assessed by SEM examination of the wear surfaces and cross-sections through the wear scars and understood in terms of the different coating microstructures.

Sliding wear behaviour of HVOF sprayed WC-Co coatings deposited with both gas-fuelled and liquid-fuelled systems

Abstract WC–Co thermally sprayed coatings are now widely used in a range of industries to combat wear. During spraying of WC–Co coatings, there is a decomposition involving dissolution of the hard phase into the molten binder; as such, the coatings are often made up of WC along with other carbides in a matrix consisting of tungsten and carbon dissolved in the cobalt. High velocity oxy-fuel (HVOF) spraying is seen as the pre-eminent process for deposition of such coatings. There have been moves in the industry to spray coatings with liquid-fuelled systems rather than gas-fuelled guns, since the former lead to shorter residence times of the particles in the flame and lower temperatures, both of which lead to less dissolution of WC and decomposition of the coating. This paper presents work concerning the sliding wear behaviour of WC–Co coatings HVOF sprayed with both liquid-fuelled (HVOLF) and gas-fuelled (HVOGF) systems and demonstrates that with a dense powder feedstock, the HVOGF deposited coating is superior to the HVOLF deposited coating. The poorer performance of the HVOLF-sprayed coating is associated with mechanical damage to the WC–Co powder particles as they impact with the substrate resulting in carbide cracking and a reduction in the integrity of the bond between the carbide particles and the matrix phase.

The effect of small stresses on the kinetics of the bainite transformation

Experiments have been conducted in which bainite is allowed to grow under the influence of an externally applied stress of magnitude less than the yield strength of austenite. By monitoring the transformation strains along two orthogonal directions, it has been possible to demonstrate that the stress in general accelerates the overall transformation kinetics, particularly at high transformation temperatures, where the chemical driving force for transformation is relatively small. An acceleration of transformation at temperatures just above the martensite start temperature has been confirmed, even in the absence of an externally applied stress. The results are interpreted in terms of the mechanism of the bainite transformation in steels.