| name | tribological-coatings |
| description | Tribological coatings — PVD (TiN, TiAlN, DLC), CVD, thermal spray, friction coefficients, hardness, adhesion, applications in cutting tools, bearings, forming dies. |
| metadata | {"priority":7,"promptSignals":{"phrases":["tribological coating","PVD coating","DLC coating","TiN coating","hard coating","wear coating","surface coating tribology"],"minScore":3}} |
Tribological Coatings — Complete Skill
Coating Types Overview
Physical Vapor Deposition (PVD)
Process: Target material evaporated/sputtered in vacuum; deposited on substrate
Temperature: 200–500°C (substrate); low temp → no distortion of hardened steel
Thickness: 1–10 μm typical
Excellent adhesion to steel, carbide; high hardness; compressive residual stress (good for fatigue)
| Coating | Hardness [GPa] | μ (dry steel) | Max temp | Notes |
|---|
| TiN (Titanium Nitride) | 20–25 | 0.4–0.6 | 550°C | Gold color; general purpose |
| TiCN (Ti Carbonitride) | 25–35 | 0.2–0.4 | 400°C | Silver-gray; better wear |
| TiAlN (Ti-Al Nitride) | 28–35 | 0.4–0.6 | 700–900°C | Best for high-speed dry cutting |
| AlTiN | 30–40 | 0.4–0.7 | 800°C | Higher Al for oxidation resist |
| CrN (Chromium Nitride) | 18–25 | 0.35–0.5 | 650°C | Corrosion + wear; die tooling |
| AlCrN | 30–35 | 0.3–0.5 | 1100°C | High-temp applications |
| WC/C (Tungsten carbide/C) | 15–20 | 0.05–0.2 | 300°C | DLC-like; low friction |
Diamond-Like Carbon (DLC)
Structure: Amorphous carbon with sp³ (diamond) and sp² (graphite) bonds
ta-C (tetrahedral amorphous): H-free; hardest DLC (40–80 GPa); FCVA process
a-C:H (hydrogenated): 15–25 GPa; very low friction (μ = 0.05–0.1 in sliding)
Properties:
Extremely low friction (μ = 0.05–0.15 dry; 0.01–0.05 lubricated)
Excellent wear resistance; chemically inert
Limitation: fails above 300–400°C (graphitization); internal stress limits thickness (< 5 μm)
Applications: engine tappets, cam followers, fuel injectors, ball bearings, biomedical implants
Chemical Vapor Deposition (CVD)
Process: Chemical reaction of gases at 900–1100°C → deposits coating
High temperatures require subsequent heat treatment (for steel tools)
Thicker than PVD (5–25 μm); excellent coverage of complex geometries
TiC (Titanium Carbide): very hard (28–35 GPa); excellent adhesion to WC-Co carbide inserts
Al₂O₃ (alumina): high-temperature oxidation resistance; ceramic layer
TiN over Al₂O₃: gold appearance; outer wear indicator
Multi-layer CVD: TiC + Al₂O₃ + TiN → standard for WC cutting inserts
Thermal Spray
HVOF (High Velocity Oxy-Fuel): WC-Co, WC-CrC-Ni; very dense coatings
WC-12Co: hardness 1000–1200 HV; excellent impact resistance
Porosity: < 1% (HVOF); 5–15% (APS plasma)
Thickness: 0.1–2 mm; good for dimensional restoration, wear protection
Plasma spray: ceramic coatings (Al₂O₃, Cr₂O₃, TBC 8YSZ)
8YSZ TBC: k = 2 W/mK; hardness 500–800 HV; thermal protection
Cold spray: powder accelerated >600 m/s; deposits without melting
Temperature-sensitive substrates; compressive stress; minimal oxidation
Adhesion Testing
Scratch test (Rockwell indenter): critical load L_c for coating delamination
L_c > 30 N (good adhesion for PVD cutting tools)
Rockwell C adhesion test: HRC indentation; examine halo under microscope
HF 1–2 = excellent; HF 3–4 = acceptable; HF 5–6 = poor → reject
Tribology of Coated Surfaces
Hertzian contact stress: coating must not crack under peak contact stress
σ_contact = E* × δ / R (Hertz) must be < coating fracture strength
Fatigue of coated surface: compressive residual stress in PVD → improved fatigue
Tensile stress in CVD → slight fatigue debit
Wear mechanisms:
Abrasive: H_coating >> H_abrasive needed (H_coating/H_abrasive > 1.5 rule)
Adhesive: DLC and CrN best (inert surface, low adhesion tendency)
Fretting: DLC or CrN recommended
Selection Guide
| Application | Coating | Reason |
|---|
| HSS drill bit | TiN or TiCN | Cost; general purpose |
| Carbide end mill (dry) | TiAlN or AlTiN | High-speed; high-temp |
| Forming die | CrN | Low friction + corrosion |
| Engine tappet | DLC (a-C:H) | Ultra-low friction in oil |
| Bearing (fretting) | DLC or TiN | Prevents fretting wear |
| Hot forging die | HVOF WC-Co | Impact + heat + wear |
Output
Provide: coating type, process (PVD/CVD/spray), hardness [GPa], friction coefficient μ, maximum operating temperature [°C], thickness [μm], adhesion test requirement, application rationale.
