SIGMA PHASE FORMATION IN A HEAT TREATED NI-۵۰CR COATING APPLIED BY HVOF PROCESS ON A STAINLESS STEEL

Abstract: Two different coating microstructures of Ni-۵۰Cr alloy were obtained on a stainless steel substrate by changing combustion characteristics of a high velocity oxy-fuel (HVOF) process and the size distribution of feed powder during coating process. Use of the finer feed powder and leaner fuel in oxygen/fuel ratio (i.e. using a ratio much less than stoichiometric ratio) led to formation of an extremely dense coating with high oxide content. Heat treating of this coating at ۶۵۰ºC for ۴ hours caused the formation of an intermetallic sigma phase having Cr۷Ni۳ stoichiometry. Formation of this phase has been reported occasionally in thin films not in thermal spray coatings, as reported for the first time in this research. In addition no sigma phase was detected in the HVOF as-deposited coating with low oxide content after heat treatment of the samples. Therefore, due to the limited number of papers available in the subject of formation of 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 of sigma phase in Ni-۵۰Cr coating deposited by HVOF technique and heat treated at ۶۵۰ºC was discussed and then the coating was characterized.

کلیدواژه ها:

sigma phase ، HVOF ، oxide phases ، thermal sprayed coatings ، Ni-۵۰Cr.