The erosion and abrasion characteristics of alumina coatings plasma sprayed under different spraying conditions

A series of plasma sprayed alumina coatings was evaluated regarding their erosion and abrasion characteristics. The coatings were deposited under different spraying conditions, using a commercial axial injection plasma spray system, and with powders of different grit sizes and crystallinity. A sintered bulk alumina and a conventionally sprayed coating, produced by a radial injection air plasma spray technique, were tested as reference materials. To evaluate the importance of energy input the coatings were produced using two different torch nozzle sizes and gas mixtures with a varied amount of hydrogen. The erosion and abrasion results indicate that hydrogen concentration, nozzle size and precursor powder type and size influence the tribological characteristics of the coatings. The wear resistance of the coatings seems to benefit from an increase in hydrogen concentration or torch nozzle size. The effect of precursor powder size on the wear resistance was more complex but indicates that sapphire powders of medium precursor sizes are advantageous to ordinary plasma spraying powders of alumina.     

Friction coefficient and wear rate of sputter-deposited thin films and plasma-sprayed thick coatings at high temperatures in room air

Silver, calcium fluoride (CaF_x with x = 1.85) and chromium-carbon (Cr₃C₂) thin films were deposited onto various tribological test specimens by sputtering. The friction properties of sputter-deposited Ag and CaF_x single layers as well as Ag/CaF_x multilayer films were determined by ball-on-disk tribological tests conducted in room air under various experimental conditions. The tribological properties (friction coefficient and wear rate) of sputter-deposited CaF_x films were also determined at 500°C by pin-on-disk tribological tests performed with pin specimens made of cobalt-based alloy (alacrite). Chromium-carbon films sputter-deposited onto alacrite disk and counterfaces were found to be of interest for reducing the formation of alacrite wear debris in the wear tracks; thus reduced friction coefficient and wear rate values were obtained. The friction behavior of sputter-deposited CaF_x/Cr₃C₂ thin bilayer structures and plasma-sprayed (PS) chromium carbide/Ag/BaF₂-CaF₂ eutectic composite coatings (PS-212 type coatings) was investigated by plane-on-plane tribological tests conducted in room air at 500°C and 700°C. The friction performance of solid lubricant thin bilayer films was compared with that of thick PS-212 type coatings similar to coatings developed by NASA.     

Development of a new type micro slurry‐jet erosion (MSE) tester for evaluation of wear properties of hard thin coatings

Versatile and reliable techniques for evaluation of hard thin coatings are necessary for the development and tribological assessment of new coatings. We have proposed a new type of micro slurry‐jet erosion (MSE) test, i.e. a solid particle impact erosion test for swift evaluation of wear properties of hard thin coatings. We are using a new type of MSE test apparatus (pot type tester) that makes it possible to obtain the wear loss per unit mass of erodent, which in this test was alumina particles with an average size of 1.2 µm. Its performance was evaluated by using a Si wafer plate under various test condition. In addition, the MSE tester was demonstrated by evaluating the wear resistance of TiN on high‐speed steel substrate. The new MSE test generates highly reproducible results and is very sensitive to the quality of the coatings. Copyright © 2009 John Wiley & Sons, Ltd.     

Sliding wear characteristics of plasma-sprayed Al2O3 and Cr2O3 coatings against copper alloy under severe conditions

Al₂O₃ and Cr₂O₃ coatings were deposited by atmospheric plasma spraying and their tribological properties dry sliding against copper alloy were evaluated using a block-on-ring configuration at room temperature. It was found that the wear resistance of Al₂O₃ coating was superior to that of the Cr₂O₃ coating under the conditions used in the present study. This mainly attributed to its better thermal conductivity of Al₂O₃ coating, which was considered to effectively facilitate the dissipation of tribological heat and alleviate the reduction of hardness due to the accumulated tribological heat. As for the Al₂O₃ coating, the wear mechanism was plastic deformation along with some micro-abrasion and fatigue-induced brittle fracture, while the failure of Cr₂O₃ coating was predominantly the crack propagation-induced detachment of transferred films and splats spallation.