Plasma Cladded Stellite Alloy High-Temperature Friction and Wear Characteristics Study

Literature Overview

This study investigates the high-temperature friction and wear behavior of plasma cladded Stellite alloy coatings, focusing on the temperature-dependent transition of wear mechanisms and the role of oxide scale formation in the tribological response. Stellite alloys (Cobalt-Chromium-Tungsten based) are widely used in high-temperature wear applications such as gas turbine components, hot section engine parts, and high-temperature valve seats. The plasma cladding process offers superior microstructural control compared to conventional welding methods, producing coatings with finer grain structures and reduced dilution.

Microstructural Characterization of Plasma Cladded Stellite Coatings

The plasma cladded Stellite 6 coating (Co-28Cr-6W-5Fe-4Ni-1Si-1C) exhibits a complex multiphase microstructure consisting of:

Matrix: Co-based solid solution enriched with Cr, W, Fe, and Ni
Primary carbides: M7C3 (Co, Cr, W, Fe) with a lamellar morphology along dendrite boundaries
Secondary carbides: M23C6 (Cr-rich) and M6C (W-rich) dispersed within dendrite cores
Intermetallics: B-phase (Co2W) and M-phase (Co3W) in W-rich regions

Microstructural Comparison: PTA vs. Conventional Welding

Feature	PTA Cladded	Conventional Arc Welded
Grain size	20-50 μm	80-200 μm
Carbide morphology	Fine, dispersed	Coarse, continuous network
Dilution with substrate	2-5%	10-25%
Hardness (HV0.3)	420-460	350-400
Carbon content	0.8-1.2%	0.5-0.9%
Cr depletion at interface	15-25 μm	40-80 μm

The finer microstructure of PTA cladded coatings provides superior wear resistance at all temperatures, but the advantage is most pronounced at elevated temperatures where the coarser conventional weld coatings undergo accelerated carbide coarsening and phase transformation.

High-Temperature Friction and Wear Behavior

Wear Rate as a Function of Temperature

Test Temperature (°C)	Wear Rate (mg/N·m)	Friction Coefficient	Dominant Wear Mechanism
25 (RT)	0.18	0.65	Mild abrasion
200	0.22	0.68	Abrasion + oxidation
400	0.35	0.72	Oxidative abrasion
600	0.55	0.78	Severe oxidation
800	0.45	0.70	Protective oxide formation
1000	0.60	0.75	Oxidative + adhesive

The wear rate exhibits a non-monotonic behavior with temperature, showing a minimum at 800°C. This is attributed to the formation of a protective Cr2O3 and Cr3O4 oxide scale that reduces the friction coefficient and protects the underlying coating from further oxidation. Below 600°C, the oxide layer is thin and non-protective, leading to increased oxidative wear. Above 800°C, the oxide scale becomes thicker and more porous, allowing oxygen diffusion to continue and causing increased wear.

Oxide Scale Composition at Different Temperatures

Temperature (°C)	Primary Oxide	Secondary Oxide	Scale Thickness (μm)	Adhesion
200	CoO	Cr2O3	0.5-1	Good