Surface Engineering and Failure Analysis
This domain combines two skills: specifying a surface treatment that delivers a required case property, and reconstructing a failure from its fracture surface and service history.
Surface treatment rules
- Carburising: 850–950 °C in a carbon-bearing atmosphere, followed by hardening and tempering; produces a high-carbon case with a tough core, and leaves beneficial compressive residual stress.
- Nitriding: 500–550 °C in ammonia gas or plasma; no quench is needed because the nitride case forms directly, giving excellent wear resistance with minimal distortion.
- Shot peening induces compressive residual stress in the surface layer, raising fatigue life by suppressing crack initiation and early growth.
- PVD operates at lower temperatures with line-of-sight deposition (a few micrometres); CVD runs hotter but coats internal surfaces and recesses better. Thermal spray builds thick coatings; electroplating is aqueous and hydrogen-charging risk exists.
Failure analysis sequence
Collect background and service history, preserve the fracture surface, do visual and macroscopic examination, then non-destructive testing, then sectioning away from the fracture, then SEM fractography, metallography and mechanical/chemical tests, and finally a written synthesis with recommendations.
Fractographic signatures
Beach marks are macroscopic arrest lines; striations are microscopic and each one represents a load cycle. Dimples indicate ductile overload, cleavage shows river patterns, and intergranular facets point to hydrogen embrittlement, temper embrittlement or SCC.
Worked example
A shaft shows striations spaced 0.2 µm apart with a fatigue zone 2 mm deep. Cycles = 2 mm / 0.2 µm = 10,000 cycles of propagation. Comparing that with 800,000 total service cycles shows the majority of life was spent initiating the crack - so surface finish and residual stress, not bulk strength, are the levers.
Traps that cost marks
Calling beach marks striations. Declaring a single mechanism from one feature. Skipping the final overload zone quantification, which would have given the failure stress.