Dry Abrasive Wear Research Articles

Microstructure and property of Al 2O 3 coating microplasma-sprayed using a novel hollow cathode torch

Al 2O 3 coating is deposited using a novel microplasma torch of a hollow cathode through axial powder injection under low power, up to several kilowatts. The microstructure of the coating is characterized using optical microscopy, scanning electron microscopy, and X-ray diffraction analysis. The property of the coating is characterized by dry rubber wheel abrasive wear test. The velocity of in-flight particles is measured using a velocity/temperature measurement system for spray particles based on thermal radiation. The effect of plasma power on particle velocity during spraying, and the microstructure and property of the coating are examined. The comparison of the velocity of in-flight particles, and microstructure and property of the coating deposited by the microplasma spraying with those of conventional plasma spraying is carried out. The results reveal that Al 2O 3 coating, of comparable performance to that deposited by conventional plasma jet under a power of about 40 kW, can be deposited by the present microplasma spray torch.

Developing diamond MMCs to improve durability in aggressive abrasive conditions

Metal Matrix Composites (MMCs) are seeing increased use in tribological applications where hardness, toughness and wear resistance are required. Offshore drilling tools are such an application. They are subjected to wear and corrosion processes, which can occur simultaneously. Having already demonstrated the feasibility of including diamond as the hard phase in MMCs through thermal spraying [Surf. Eng., submitted for publication], this paper focuses on durability issues relating to the coatings. Synthetic, coated and uncoated diamond grits were used in conjunction with nickel-based hard facing powders as a matrix, to produce composite coatings. The coatings were then investigated in terms of their microstructure (light microscope, Scanning Electron Microscope (SEM)), their elemental composition using EDX and XRD to identify retention of diamond and phases in the coatings, their hardness and their resistance to dry abrasive wear. Preliminary results show that it is possible to produce a hard facing diamond composite coating with good distribution of the diamond phase and little degradation of the diamond during the spraying. The coatings show some limitations in terms of their quality and homogeneity but the work presented in this paper demonstrates that diamond MMCs (DMMCs), produced by thermal spraying, offer potential as durable coatings.

Residual stresses in aluminium phosphate sealed plasma sprayed oxide coatings and their effect on abrasive wear

Effect of residual stresses on plasma sprayed alumina and chromia coatings sealed with aluminium phosphate were studied as a function of the temperature of the sealing treatment. Stresses were measured by X-ray stress analysis and high-speed circular microhole drilling method. Residual stress states were correlated with other coating properties such as microhardness, porosity, microstructure and dry abrasion wear resistance. Correlations were found between sealing treatment temperature, residual stress state and wear resistance. Wear resistance of the oxide coatings was increased at all sealing temperatures. Sealing treatment affected coatings by two mechanisms. Aluminium phosphate sealing induced compressive stresses to coatings and simultaneously bonded coating lamellar structure.

Thermally sprayed Ni(Cr)–TiB 2 coatings using powder produced by self-propagating high temperature synthesis: microstructure and abrasive wear behaviour

High velocity oxy-fuel (HVOF) thermal spraying was used to deposit Ni(Cr)–TiB 2 cermet coatings onto steel substrates from a feedstock powder produced by a self-propagating high-temperature synthesis (SHS) reaction. The powder and coating microstructures were investigated by scanning electron microscopy and X-ray diffraction (XRD), whilst dry sand rubber wheel abrasive wear and microhardness tests were performed on the coatings. The SHS reacted powder was found to comprise principally 1–5 μm sized TiB 2 in a crystalline, Ni-based solid solution matrix, with approximately 60 vol.% TiB 2 and 40 vol.% metallic binder. XRD revealed the presence of only around 2% of the following phases: TiB, Ni 2B, NiTi, TiO. The reacted compact was crushed, ground and classified into a powder of size range (+8 to −38) μm for thermal spraying. The HVOF sprayed deposits had layered, splat-like morphologies typical of thermally sprayed cermets. The coating microstructure consisted primarily of TiB 2 in the Ni-rich binder phase but the boride fraction was reduced compared to that of the feedstock powder as a result of TiB 2 dissolution in the molten alloy during spraying. XRD analysis of the coating indicated that the binder solidified to, in part, an amorphous/nanocrystalline structure with ∼2% of Ti-containing oxide phases also present. The TiB, Ni 2B and NiTi phases were not detected in the coating. Under the wear conditions employed, angular alumina generated significantly higher wear rates than the rounded silica abradent, and the wear mechanism changed from microcutting with alumina to pull-out with silica. The wear coefficients obtained with the coating were significantly reduced compared with those of mild steel, in the case of alumina abrasive a factor of four reduction. Additionally, a significant improvement in wear resistance was also found when compared to HVOF-sprayed chromium carbide–nickel/chromium cermet coatings produced from commercially available powders.

Microstructure and Properties of Pressure Die Cast Aluminium/Zirconium Silicate Composites

Al-7.3Si-2.4Mg alloy matrix composite dispersed with 15 mass%, zircon sand (zirconium silicate, ZrSiO 4 ) particles of 50 μm average particle size (APS) (20-200 μm size range) synthesized by stir casting was pressure die cast into plates. The zircon sand particles were uniformly distributed in the pressure die cast composite plates with refined Si, Mg 2 Si and other phases along with finely dispersed gas porosities compared to gravity die cast composites. The ultimate tensile strength of the pressure die cast composite plates (195-205 MPa) were superior to both gravity die cast composite (70 MPa) and pressure die cast matrix alloy (145 MPa) plates. The tensile fracture of pressure die cast composite showed particle cracking suggesting existence of good interfacial bonding between the particle and the matrix. The tensile properties of the pressure die cast composites at temperatures exceeding 423 K decreased sharply as a result of opening up of entrapped gas porosities. The dry abrasion wear tests showed only removal of the matrix and dispersed zircon sand particle offering resistance to wear when the abraded surface particle sizes were smaller (40 μm) than the zircon sand particles (50 μm) in the composites. On the other hand when the abrading surface had larger particles (400 μm) extensive wear of the composite took place by breaking of the zircon sand particles as well as by removal of the matrix.

Improving abrasion wear by surface treatment

An increase in the abrasive wear resistance of hardened and tempered hot-work die steel can be achieved by the use of surface treating processes, such as nitrocarburizing, plasma nitriding and spark discharge cladding, which produces a hard surface layer of WC-Co alloy. The dry abrasive wear test used in this investigation takes place in a dry air atmosphere and uses ferric oxides as the abrasive to form a range of surface topographies. In this test, the smooth surface of each specimen is subjected to rolling, and this results initially in groove and scar formation; subsequently the surface roughens and substantial amounts of debris are produced. The test results show that there is a linear relationship between the wear weight loss and the wear time. Metallography and X-ray diffraction, respectively, were used to examine the wear surface and the debris. It is clear that the abrasive wear resistance of a hot-work die steel can be improved by means of these types of surface treatments, most notably plasma nitriding. The phenomenon of abrasive wear is explained in terms of the wear test and metallographic results and the wear mechanism for these conditions is postulated.

Plasma Arc Carbide Coatings on Titanium

ABSTRACTThe plasma transferred arc (PTA) process has been traditionally used to deposit wear resistant coatings on iron base alloy substrates, but has not been employed to coat lightweight alloys due to processing problems. In the current study, use of the PTA process to deposit TiC, WC, and Cr3C2 coatings on titanium substrates has been explored. The resistance of these coatings to dry abrasive wear has been measured and compared to that of the base metal. The variation in wear resistance of these coatings is discussed in terms of the carbide particle size and the microstructure of the deposit. A comparison is also made to coatings prepared by a laser surface melting and carbide particle injection process.