In a precision mechanism the failure is rarely dramatic. A spindle that should turn without play develops a fine wear track, a shaft running in a hydraulic circuit weeps at the seal, a guide rail picks up a polishing pattern that changes the position accuracy of the machine. High-precision component coating by PVD addresses a specific problem: a surface that has to hold controlled friction and wear behaviour over millions of cycles, at a film thickness small enough not to disturb the fit.
That last point separates precision work from general wear protection. At 1-3 μm the film is usually smaller than the clearance it sits in, but not smaller than the tolerance on a P4 bearing or the positional tolerance on a shaft journal. Film thickness, roughness and the resulting dimensional change are therefore part of the coating specification, not a by-product.
Typical parts we coat
- Bearings: spindle, high-speed, sliding and hydrodynamic bearings, turbine and turbocharger bearing running surfaces
- Shafts and journals: motor, transmission and pump shafts, valve stems, mandrels, arbors and precision spindles
- Linear and feed motion: guide rails and linear guide components
- Rotary and sliding joints: rotary unions, needle valve stems, eccentric rods and sliding bearing shells
- Power transmission: gears, gear pins, robot joint parts, hinges and pin-joint components
- Precision fasteners and electrical contacts: high-strength screws and nuts, copper busbars, terminals
Coating options and why they fit
DLC (diamond-like carbon)
DLC is the primary choice wherever friction has to come down and the sliding speed is high. A dry coefficient of friction around 0.1 is typical, and under boundary lubrication friction losses typically fall by 10-30 %. The film is thin, typically 1-3 μm, so it does not disturb a running clearance. DLC needs a hard supporting layer and a clean substrate, and its adhesion is sensitive to pre-treatment.
Chromium nitride (CrN)
CrN is the balanced option where wear, corrosion and moderate temperature all matter. Typical Vickers hardness of 1800-2200 HV, a friction coefficient that can be held below 0.3 on a treated journal, and a wear rate reported at 4-8 times lower than the tempered steel base material. CrN is stable in air up to about 600 °C, which suits turbine bearing running surfaces, and it is the usual choice on PH steel components that must resist both wear and a damp or aggressive environment. It is also the answer where corrosion protection matters: uncoated bearing steel has been observed to show rust in neutral salt spray at 48-96 h.
Titanium nitride (TiN) and titanium carbonitride (TiCN)
TiN remains the economical choice for general wear protection on steel, with a typical Vickers hardness of 2200-2500 HV. It is widely used on precision fasteners, where the useful result is lower friction torque during assembly. On a mechanical joint, a TiN film has been measured at 2200-2800 HK in microhardness, with friction falling from 0.35 to 0.15-0.25 and a wear rate 3-10 times lower. TiCN is the harder variant, for higher loads.
Nickel-based hard layers
Nickel compound layers are used where adhesion, wear resistance and corrosion resistance are all needed on a shaft or mandrel. One reported application used a hard gradient from 1500 HV at the interface up to 2200 HV at the surface, a roughness of Ra 0.1 μm or better, and a friction coefficient of 0.25.
How the process works
The route is pre-treatment, ion bombardment, deposition and post-treatment. On precision components pre-treatment is where most of the tolerance risk sits: cleaning, and for journals and rails a check of surface finish and geometric form before fixturing. Ion bombardment then removes the last oxide and activates the surface so that the film bonds metallurgically rather than mechanically. Arc ion plating is used for most hard wear films because of the plasma wrap and dense structure; magnetron sputtering where the roughness specification is tight. Post-treatment covers cooling, a dimensional check against the drawing, and roughness and runout checks.
Deposition temperature has to be kept below the tempering temperature of the base material. Bearing steel work is typically run at 150-300 °C with a film of 1-3 μm, while general component work runs at 200-450 °C with a film of 2-6 μm. A base vacuum below 5×10⁻³ Pa and a working pressure of 0.5-2 Pa are typical, and a substrate bias of -50 to -200 V densifies the film and drives adhesion. Films are measured after coating by X-ray fluorescence and ellipsometry in cross-check.
What the numbers look like
- DLC on a sliding bearing: typical dry coefficient of friction around 0.1, boundary-lubricated friction loss typically 10-30 % lower
- DLC on a high-speed bearing in oil lubrication: friction torque typically 15-40 % lower, L10 fatigue life typically 2-4 times longer
- CrN on a shaft journal: friction typically below 0.3, wear typically 4-8 times lower than tempered steel
- CrN on a bearing component: stable in air to about 600 °C, rolling contact fatigue life typically 2-4 times the uncoated part
- CrN in water-treatment service: neutral salt spray typically over 500 h without substrate red rust for a 3-5 μm film, against 96-200 h for plated chrome
- Untreated bearing steel in neutral salt spray: typically shows rust at 48-96 h; a coated part extends that
- TiN on a high-strength fastener: typical Vickers hardness of 2200-2500 HV, friction torque typically 30-40 % lower, fastener life typically 3-5 times longer, salt spray typically 400 h against 100 h for zinc plating
- Titanium nitride on a mechanical joint: typical microhardness of 2200-2800 HK, friction typically reduced from 0.35 to 0.15-0.25, wear rate typically 3-10 times lower
- Nickel compound layer on a mandrel: typical Vickers hardness of 1900-2400 HV, scratch critical load typically above 55 N, friction typically 0.3-0.5
- Chromium nitride on a copper busbar: film thickness typically 1.0-3.0 μm, microhardness typically 1200-1800 HV, porosity typically below 0.5 %
Where this fits — and where it doesn’t
- Fits: components whose geometry is correct and whose failure is friction, wear or a corrosion product in a clearance — sliding and rotating surfaces, precision fasteners, and electrical contacts needing a stable surface.
- Fits: applications where the film has to be thin enough to leave a running clearance unchanged. At 1-3 μm this is usually automatic, but confirm it against the drawing.
- Does not fit: high-precision bearings and fits already at their tolerance limit. The coating changes the clearance and the roundness of the race, so P4 or P2 class bearings need re-measurement and a new clearance group.
- Does not fit: parts whose surface condition is already the problem. A worn journal, scored rail or pitted seal face carries its defect into the film.
- Does not fit: high-strength steel parts where hydrogen embrittlement is a risk and the coating route has not been reviewed against the material specification. A low-temperature route, which limits hydrogen uptake, is used here and the process is agreed in advance.
Frequently asked questions
Does the coating change the bearing clearance?
By the film thickness. A film of 1-3 μm is smaller than most running clearances, so no adjustment is normally needed. On a very high-precision bearing the coating still changes the radial clearance and race profile, so the clearance group has to be re-selected from measured values and the vibration figure re-checked.
How accurately can the film thickness be held?
Film thickness is measured after coating using X-ray fluorescence and ellipsometry in cross-check, with a thickness control typically within ±5 %. Where the function only needs a conductive skin, a chromium or nickel layer of 0.05-0.5 μm is used, with a surface roughness of Ra 0.2 μm or better.
Can a shaft be coated after it has been in service?
Only if the base condition allows it. A shaft with a measurable wear step, score or rust pit should be re-ground or replaced first, because the coating reproduces the geometry it is given. A shaft within drawing tolerance but showing light scoring can often be coated after a polish.
Send us a sample for a free coating feasibility assessment. The material, drawing tolerances, running clearance and lubricant are enough to start, and a used part with its wear history tells more than a drawing.
Xi’an Zhiyang Baina Vacuum Coating Co., Ltd. | PVD vacuum coating services and coating technology cooperation for all types of parts. Samples welcome for a free coating assessment.
Address: Xi’an, Shaanxi, China