Suspension High Velocity Oxy-Fuel (SHVOF)-Sprayed Alumina Coatings: Microstructure, Nanoindentation and Wear

Abstract

Suspension High Velocity Oxy Fuel Spraying (SHVOF) can be used to produce thermally sprayed coatings from powdered feedstocks too small to be processed by mechanical feeders, allowing formation of nanostructured coatings with improved density and mechanical properties. Here, alumina coatings were produced from sub-micron sized feedstock in aqueous suspension, using two flame combustion parameters yielding contrasting microstructures. Both coatings were tested in dry sliding wear conditions with an alumina counterbody. The coating processed with high combustion power of 101 kW contained 74 wt% amorphous phase and 26 wt% crystalline phase (95 wt% gamma and 3 wt% alpha alumina) while the 72 kW coating contained lower 58 wt% amorphous phase and 42 wt% crystalline phases (73 wt% was alpha and 26 wt % gamma). The 101 kW coating had a dry sliding specific wear rate between 4-4.5 x 10 -5 mm 3 /Nm, 2 orders of magnitude higher than the 72 kW coating wear rate of 2-4.2 x 10 -7 mm 3 /Nm. A severe wear regime dominated by brittle fracture and grain pull out of the coating was responsible for the wear of the 101 kW coating, explained by mean fracture toughness three times lower than the 72 kW coating, owing to the almost complete absence of alpha alumina.

Suspension high velocity oxy-fuel (SHVOF) spray of delta-theta alumina suspension: Phase transformation and tribology

“…Thermally sprayed ceramic coatings are used to protect engineering components against severe wear and corrosion in a wide range of applications e.g. bearings, aircraft, automotive engines and agricultural machinery [1][2][3][4][5][6]. Alumina is widely used for its reduced cost, wear resistance, stability at high temperature, good dielectric strength and resistance against chemical attack [4,7].…”

“…Nano-structured, alumina based ceramic coatings have benefits over their conventional counterparts including enhanced adhesive strength (~65-70 MPa) [9], resistance to crack growth [10,11], and excellent wear performance [12][13][14]. These coatings have been extensively studied in suspension feedstock studies using a modified HVOF thermal spray setup [6,9,[15][16][17]. Suspension HVOF (SHVOF) thermal spraying is a variant of HVOF thermal spraying that offers the capability to deposit coatings with fine microstructures from nano to submicron scale liquidised feedstock.…”

“…The microstructure of SHVOF alumina coatings have been studied with regards to feedstock composition [24] and processing conditions [6,9,18]. The microstructure of SHVOF coatings was shown to be affected by particle size distribution in the suspension and processing parameters which in turn affect the coating density, thickness and the constituent phases.…”

“…The microstructure of SHVOF coatings was shown to be affected by particle size distribution in the suspension and processing parameters which in turn affect the coating density, thickness and the constituent phases. The predominant phase in SHVOF sprayed alumina is typically γ-Al2O3 with some amorphous phase depending on the initial feedstock and/or processing conditions [6,9,18]. Coatings with higher phase fractions of γ-Al2O3 are typically homogenous, less porous and have good coating-substrate bonding.…”

Microstructure, Wear Behavior and Corrosion Resistance of WC-FeCrAl and WC-WB-Co Coatings

“…Thermally sprayed coatings are often used to increase the resistance of the parent material against corrosion and wear [3][4][5][6][7][8][9][10][11][12][13][14][15]. The desired properties of coatings applied by the high-velocity oxygen fuel (HVOF) method also include high density and good adhesion, which are characterized by the ability to form coatings with a small portion of oxides and phase transformations [16][17][18][19].…”