This research aims to characterise and quantify the acoustic emission (AE) generated
during the high velocity oxy-fuel (HVOF) thermal spraying process, recorded using piezoelectric
AE sensors. The HVOF process is very complex involving high temperature turbulent flow through
a nozzle with entrained particles, the projection of these particles, and their interaction with the
target surface. Process parameters such as gun speed, oxy-fuel pressure and powder specification
affect various characteristics of the coating, including thermal residual stresses; the lamellar
microstructure and the topology and geometry of pores, all formed when the fused powder hits the
surface, forming “splats”. It is widely acknowledged in the thermal spray industry that existing
quality control techniques and testing techniques need to be improved. New techniques which help
to understand the effects of coating process parameters on the characteristics of the coating are
therefore of value, and it was anticipated that recording the AE produced when the fused particles
contact the surface would aid this understanding. As a first stage, we demonstrated here that AE
associated with particle impact can, in fact, be discerned in the face of the considerable airborne and
structure-borne noise.
In order to do this, a new test method using a masking sheet with slits of varying size was
developed. Thermal spraying was carried out for a range of spray gun speeds and process
parameters. The AE was measured using a broad band AE sensor positioned on the back of the
sample as the spot was traversed across it. The results show that the amplitude and energy of the
AE signals is related to the spray gun speed, powder used and the oxy-fuel pressure. Using a simple
geometrical model for particle impact, the measured AE was found to vary with the energy and
number of particles impacting on the sample in a predictable way.
Investigations on Corrosion Behaviour of WC–CrC–Ni Coatings Deposited by HVOF Thermal Spraying Process