SIGMA PHASE FORMATION IN A HEAT TREATED NI-50CR COXTING APPLIED BY HVOF PROCESS ON A STAINLESS STEEL

Two different coating microstructures of ni-50cr alloy were obtained on a stainless steel substrate by changing combustion characteristics of a high velocity oxy-fuel (hvof) process and the size distribution offeedpowder during coating process. use of the finer feed powder and leaner fuel in oxygenljuel ratio (i.e. using a ratio mucl less than stoichiometric ratio) led to formation of an extremely dense coating with high oxide content. heat treat.ng of this coating at 650°c .for 4 hours caused the formation of an intermetallic sigma phase having cr7ni3 stoichiometry. formation ofthis phase has been reported occasionally in thin films not in thermal spray coatings. as reportedfor thefirst time in this research. in addition no sigma phase was detected in the hvof as-deposited coating with low oxide content after heat treatment ofthe samples. therefore, due to the limited number ofpapers available inthe subject offormation of 0 phase in either ni-cr bulk alloys or coatings, it is considered appropriate to show up a case in this field. in this work, the formation ofsigma phase in ni-50cr coating deposited by hvof technique and heal treated at 650°c was discussed and then the coating was characterized.