tribological-coatings

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.