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TECHNICAL REPORTS Wear-Reducing Technologies for Rotary Compressors Using CO2 Refrigerant 1. Introduction We employed a natural CO2 refrigerant with a high operating pressure in a single rotary compressor. By coating the vane, for the first time in Mitsubishi rotary compressors, we intended to reduce the wear between the top of the vane and the outside of the rolling piston. 1. Application of a Coating to the Vane Household heat pump water heaters not only use a natural CO2 refrigerant, which has a high operating pressure, but also require a longer product lifetime than air-conditioning systems or refrigerating machines. In the early stages of development of the compressor, we used vanes prepared by nitriding high-speed steel in the same manner as conventional rotary compressors used for air-conditioning systems with the result that abnormal wear occurred at the top of the vane after operating for less than one-fifth of the lifetime needed. Then, we applied a DLC-Si to the vane, which is a coating of diamond-like-carbon containing silicon. 2. Characteristics of the DLC-Si DLC-Si is accomplished with a plasma CVD (Chemical Vapor Deposition) method in which the coating forms in a plasma environment created by mixing a hydrocarbon gas and a gas containing silicon. With silicon contained in the film, the coating formed is thicker than general DLC, thus achieving better sliding and rolling characteristics. Since the top of the vane of the rotary compressor and the outside of the rolling piston make line contact, the stress becomes severe and the contact involves both sliding and rolling phenomena. When applying the coating to the vane, the adhe- sion strength between the coating film and the base material must be adequate, otherwise, the coating film may peel off and result in abnormal wear. It is neces- sary to identify the adhesion strength between the coating film and the base material for the appropriate application of the DLC-Si on the vane of a rotary com- pressor. We measured the adhesion strength with a micro scratch tester. Figure 1 shows the relationship between the silicon concentration in the DLC-Si film and the adhesion strength between the coating film and the base material. According to Fig. 1, it is clear that the adhesion strength between the DLC-Si film and the base material de- pends on the silicon concentration in the coating and is highest at a silicon concentration of approximately 20 wt%. It is also reported that the adhesion strength of the coating film depends on the internal stress developed at the interface and adhesion strength increases when internal stress is controlled properly.(3) Authors: Hideto Nakao* and Naotaka Hattori** Silicon concentration in the DLC-Si film, wt% Fig. 1 The relationship between silicon concentration and the adhesion strength ratio of DLC-Si In accordance with our examination results on the relationship between the silicon concentration in the film and the internal stress of the DLC-Si film applied to the vane of the rotary compressor, we confirmed that the lower the silicon concentration, the higher the internal stress, provided that the silicon concentration is 20 wt% or lower. This generally agrees with the reported char- acteristics mentioned above. 3. Structure of the Experimental Apparatus and Conditions of the Elemental Test Table 1 shows the conditions of the wear test on the vane and rolling piston using the experimental apparatus. Table 1 Conditions of elemental tests Environment Environment temperature Lubricant Vane material Rolling piston material Pressure loading Sliding velocity CO2 313K PAG (VG-100: dropping) High-speed steel (nitriding treatment) High-speed steel (DLC-Si coated) Special cast iron 450N 0.6m/s *Advanced Technology R&D Center **Mitsubishi Electric Engineering Company Limited Mitsubishi Electric ADVANCE December 2007 13 Ratio of adhesion strength of the DLC-Si filmPDF Image | Heat Pump with Natural Refrigerants 3041
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