Yang Alternatives to hard chromium plating on piston rods Degree Project of 30 credit points
As a main part of hydraulic cylinder, the piston rod is used in tough surrounding and corrosive condition; consequently a high quality protecting layer is essential. Currently EHC, electroplating hard chromium (hard chrome) is a widespread method for piston rod due to powerful performance and low cost. Unfortunately the plating process contains hexavalent chromium (Cr) which is toxic and carcinogenic; perhaps it will be prohibited in the nearing future. Therefore it is crucial to seeking alternatives to hard chrome for piston rod manufacturer. Through extensive searching for alternatives to hard chrome, the competitors like HVOF, electroplating tungsten plating and stainless steel linear were studied in the present work. Since microhardness and corrosion resistance are top priorities, after comparison perhaps Cr3C2-NiCr20% HVOF is the most promising technique. Distinct pretreatments Cr3C2-NiCr coatings were compared with hard chrome in pendulum impact test, hardness test and acetic acid salt spray (AASS) test. The results indicate that hard chrome show higher toughness more than 40% compare with HVOF, the microhardness of HVOF is higher than hard chrome, mean value 1784 Vs. 1244 HV and both coatings have excellent bonding strength. After 40 hours in AASS test, the hard chrome was rated No. 10, however the HVOF just No. 6.
HVOF coatings as an alternative to hard chrome for pistons and valves
Automotive manufacturers have specified chromium plating for decades because of its appearance, wear and corrosion resistance, however chromium plating cause effects on human health because of the use of substances in the galvanic process whose toxicological features have not always been recognised. The improvements of the high-velocity oxy-fuel thermal spray process allow the chromium coating replacement with a comparable or superior surfaces and more environment friendly. The present study describes and compare the mechanical and tribological properties of HVOF CrC75 (NiCr20) 25 coatings sprayed from three different agglomerated feedstock powders with various powder size distributions. These results have been compared with conventional hard chromium plating. The objective of the present work is applying these new HVOF coatings in piston rings and valve stems applications. Furthermore, the studied HVOF coatings are produced with fine-powders in order to avoid the blasting and regrinding operations necessary when plasma spray coatings are used. The coating microstructures were characterised by BSE–SEM microscopy. Differences in roughness have been determined by profilometry. The ultra-microindentation technique was applied to measure the hardness and the elasto-plastic properties of the coatings. Experiments using a pin on disc tribometer under lubricated and dry conditions have been performed in order to evaluate the friction and wear properties of the different coatings. It was found that the CrC–NiCr coating, obtained with the lowest feedstock powder size, presented the best wear resistance under all the studied conditions. The Fine CrC–NiCr coatings have demonstrated superior performance to hard chrome with regard to mechanical and tribological properties, and they can be proposed as an alternative to hard chrome coatings.
Effect of WC Particle Size on the Abrasive Wear of Thermally Sprayed WC-Co Coatings
Abstract Four types of WC-Co powders with different WC particle sizes are sprayed to produce WC-Co coatings with a Jet-Kote gun using a gas mixture of C2H2–30% C3H6 and pure propylene as fuel gases. The carbide size in sprayed high velocity oxy-fuel (HVOF) WC-Co coatings is measured from the microstructure. The abrasive wear of the coatings is characterized using a Suga abrasive wear tester. The abrasive wear mechanism of thermally sprayed WC-Co is discussed. The relationship between the relative wear of WC-Co coatings and carbide sizes is established according to the strength theory on hard alloy. The correlation with experimental data proves that the relative abrasive wear of WC-Co coating is proportional to square root of relative carbide size.