Surface wear study of composite spray coated steel I. S. Jalham* and A. Alian In this investigation, a comparative study among different types of coating materials to improve the wear resistance of steel was conducted. There were three types of coating material, magnesium zirconate, aluminium bronze, and molybdenum, and mixtures of different percentages of them were used. The influences of load, velocity and counter surface roughness on each type were also determined. It was found that the addition of molybdenum to magnesium zirconate in a sufficient amount helped to increase its wear resistance in comparison to the wear behaviour of pure magnesium zirconate. Scanning electron microscopy and microhardness testing of longitudinal cross-sections of each of the materials were used to explore the reasons behind the interesting behaviour of these composites after abrasive wear testing. It was found that the reason behind the high wear resistance and high microhardness of the magnesium zirconate matrix after the addition of 10% molybdenum was the prevention of the pull out of magnesium zirconate particles. The abrasive wear mechanisms for the spray coated steel appear to be microcracking and microploughing. Keywords: Coating, Plasma spray process, Precoating, Surface roughness, Wear Introduction Plasma spraying is a widely used technique for the production of various protective coatings which find applications as thermal barriers, 1,2 corrosion resistant layers, 3,4 and wear resistant surfaces. 5–7 These coatings are made from a broad range of materials encompassing metals, ceramics, and polymers. The process involves introduction of the material, in the form of powder, to a plasma flame, which melts the particles and propels them towards the substrate to be coated. Upon impact, the particles flatten, cool down and solidify, forming a solid layer. As a result of this process, the coatings have properties quite different from those of bulk materials of the same composition, as a consequence of porosity, anisotropy and residual stress. Abrasive wear is caused by moving particles; that is, interface elements that remove metal from the surface(s) are involved. The particles may be trapped between the two surfaces (high stress abrasive wear) or loosely packed (low stress abrasive wear); they may be present by themselves or within liquids, gases and lubricants. Wear by abrasion is probably the most damaging and costly of the wear mechanisms. Contact wear occurs when the surface and the counter surface are forced together and moved relative to each other. Under certain conditions of temperature and load, mating surfaces oxidise and control the amount of wear. Under higher loads, and without the protection of oxides, the two surfaces may adhere; this is referred to as galling, and in extreme cases results in the parts seizing or welding together. If the loading is such that the subsurface shear stresses are large enough and the load is cyclic in nature, the part may fail by subsurface contact fatigue. If the load has a vibratory component, fretting wear occurs. Erosion wear is characterised by flow, especially fluid flow. The fluid may contain particles that may be solids or gas bubbles. In some respects, erosive wear is similar to low stress abrasive wear, but it can also involve corrosion. 8 When attempts are made to improve the wear resistance of the steels by increasing the alloying content, problems can occur during manufacturing at the mill, and when alloyed steel is used in certain applications, poor cracking resistance of the alloy limits its effectiveness. These limitations have led to the development of vapour deposition tech- niques, and high energy and plasma coating processes. Some researchers 7,9 have used molybdenum for coating. It has been shown that the wear resistance of steel after molybdenum coating is dramatically improved. 7 Other researchers have used ceramic particles such as ZrO 2 and Y 2 O 3 , 1 Al 2 O 3 , 10 and NizAl 2 O 3 . 11 However, the major drawback of the ceramic components is their inherent brittleness. 12 It is clear from the literature that mainly pure ceramic materials have been used as coatings to improve wear resistance. It is also well known that brittleness is one of the most important limitations of such ceramics. Moreover, it is also noted that the influence of the roughness of the counter surface in contact with the coating material on wear resistance has not been studied extensively. Industrial Engineering Department, Faculty of Engineering & Technology, University of Jordan, Amman-Jordan *Corresponding author, email jalham@ju.edu.jo ß 2005 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 29 June 2004; accepted 5 January 2005 DOI 10.1179/174329405X55311 Surface Engineering 2005 VOL 21 NO 5–6 431