Abstract: OSHA's (Occupational Safety and Heath Administration USA) new standard for occupational exposure to hexavalent chromium (hex Cr) presents the users of thermal spray processes with both challenges and an opportunity. The new standard significantly lowers the PEL (Permissible Exposure Limit) for hex Cr from the prior level of -52 žg/m3 to 5 žg/m3 on an 8 hour time-weighted average. It also requires action, including continuously monitoring the concentration level that is above 2.5 žg/m3 and creates a requirement for manufacturers to keep below 1 žg/m3 to be reliably within the regulations. In the industry, chromium electroplating was found suitable for applications in coating of new elements and reclamation of machinery parts. Various advantages of the chromium coatings play the decisive role - high hardness of chromium coating, high abrasion resistance, resistance against chemical action, and temperature and aesthetic appearance. Depending on intended use, chromium coatings may be applied with a bath containing trivalent or hexavalent chromium. Baths with trivalent chromium are applied only for obtaining very thin decorative coatings. Solutions containing hex Cr give coatings with much better properties but creating the biggest health risk for people. Hexavalent chromium is a carcinogenic agent, as well is responsible for allergies, irritation of skin and the respiratory track with contact of skin with the solution. The mist created during this process directly affects people. More over, the technological process of chromium electroplating is the source of many harmful wastes. The above mentioned problems, as well as the introduction of new regulations in the protection of environment made it necessary to search for new technologies. Thermal spray technologies (flame and plasma) were sprayed and were applied on many machinery parts instead of chromium electroplating, particularly when effect of scuffing was common. However, their wear resistance was lower than that of chromium plated coatings, and this made it impossible for popular application. Much better properties characterise HVOF (High Velocity Oxy Fuel) sprayed coatings, introduced in the middle of the 80s. A coating in this process is formed above others from particles in a high plastic state, because they stay in the gas stream very shortly, which avoids their oxidation and phase transformation. Additionally, the high kinetic energy of particles allows for their proper deformation, although they are not in a liquid state. Hence, a coating very well bonded with the substrate with minimal porosity (less than 1%) and excellent properties is obtained. Chromium electroplated coatings and HVOF coatings show very high wear resistance, but tests on their durability are really time consuming. Tests of scuffing resistance allow one to obtain the important property of material in the hard conditions of collaboration in a shorter time. Comparative tests of scuffing resistance of both coatings under dry friction conditions were carried out with Falex T-09 tribological tester. Analysis of the microstructure of the sprayed coatings was carried out by Joel JSM-5400 SEM and EDS microprobe. The microhardness of chromium coating was 968 HV 0.1 and 1218 HV 0.1 for WC12Co coating. Surface roughness Ra was 0.11 and 0.10, respectively. It was assumed that scuffing phenomenon occurs at the moment of the truncation of the cooper pin attaching roaring counter-specimen. The loading force at which scuffing occurred was for chromium coating 268+/-98 N, and for HVOF coating 1853+/-136 N. Analysis of the loading force sequence of the chromium coated specimen shows that mating with 45 steel after heat treatment under dry friction conditions leads to scuffing from the beginning. Further decrease of the loading force indicates substantial loss of the substrate. HVOF coatings tolerates an increase in loading force very well. Degradation of HVOF coating does not occur. Under dry friction conditions, WC12
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