Surface Finish and Treatment Reference
Pick the functional requirement, not the process name: corrosion, wear, low friction, grounding, insulation, colour, hardening, fatigue, hygiene, high temperature. 28 treatments with thickness, hardness, corrosion performance, effect on conductivity, appearance, the size change each one causes, service temperature and the standard behind it. Plus the Ra and Rz series and what each machining process can actually hold.
14 requirements
28 processes
Size change per side
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Pick by requirement
The finish is a functional decision before it is a cosmetic one. Each card says what the requirement really needs, where to start, and the failure that catches people out. Click a card or use the dropdown and the process table below filters to what delivers it.
The size change column is the one engineers are caught by: anodizing, plating, powder coating and nitriding all move a dimension, while passivation, chromate conversion, PVD and laser marking leave it effectively untouched.
Process data table
Typical commercial ranges. Thickness, hardness and corrosion figures come from the process standards and supplier practice; the exact figure for your part is set on the purchase order and verified on the certificate of conformity.
| Process | Family | Substrate | Thickness | Surface hardness and wear | Corrosion | Effect on conductivity | Appearance | Size change per side | Max temp °C | Functions | Standards | Typical parts |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Anodize, Type II硫酸阳极氧化 | Anodizing | Aluminum | 5 - 25 um | 200 - 400 HV oxide; hard but brittle and chips at sharp edges | 300 - 500 h salt spray when sealed | Insulating, breakdown about 20 - 30 V per um | Clear, dyed black or any colour, satin to semi gloss | About half grows outward: roughly +10 um per side at a 20 um layer | 150 | 耐腐蚀防护 外观装饰 电绝缘 | MIL-A-8625 Type II, ISO 7599, GB/T 8013.1 | Enclosures, brackets, cosmetic panels, instrument and camera housings |
| Hard anodize, Type III硬质阳极氧化 | Anodizing | Aluminum | 25 - 60 um, up to 100 um | 350 - 500 HV; the wear answer on aluminium | 500 - 1000 h salt spray when sealed | Insulating, same order as Type II | Dark grey to bronze or black, matte | About half grows outward: +15 to +30 um per side | 200 | 耐磨 耐腐蚀防护 电绝缘 低摩擦脱模 | MIL-A-8625 Type III, GB/T 8013.1 | Hydraulic cylinder bores, valve bodies, wear plates, sliding and pneumatic surfaces |
| Anodize, chromic Type I铬酸阳极氧化 | Anodizing | Aluminum | 2 - 7 um | Thin, minimal fatigue penalty, not a wear layer | 200 - 336 h salt spray | Insulating | Grey green to clear | Half outward: +1 to +3 um per side | 150 | 耐腐蚀防护 疲劳寿命 | MIL-A-8625 Type I and IB, AMS 2470 | Aerospace structural parts where fatigue life must be preserved, bonded and riveted assemblies |
| Electroless nickel, Ni-P化学镀镍 | Plating, electroless | Steel, stainless, copper, aluminium with zincate | 5 - 50 um | 500 - 700 HV as plated; 900 - 1000 HV after 1 h at 400 C | 300 - 1000 h salt spray depending on thickness and phosphorus | Conductive, contact resistance higher than bare metal | Bright to matte silver, can be blackened | 100 percent build: 1 um up is 1 um per side, so mask or compensate | 400 | 耐磨 耐腐蚀防护 导电接地 耐高温 | ASTM B733, ISO 4527, GB/T 13913 | Hydraulic spools, actuator parts, complex internal bores, valve components, wear faces ground after coating |
| Hard chrome plating镀硬铬 | Plating, electrolytic | Steel, copper, aluminium with an undercoat | 20 - 100 um | 800 - 1100 HV; the classic wear and build-up layer | Limited alone because of microcracks, 200 - 500 h with a nickel undercoat | Conductive | Bright blue silver, satin when ground | 100 percent build, then ground back to size | 400 | 耐磨 低摩擦脱模 | ISO 6158, GB/T 11379 | Hydraulic rods, mould cores, press tools, wear rings, salvage of worn shafts |
| Nickel plating, electrolytic电镀镍 | Plating, electrolytic | Steel, copper, brass, zinc | 5 - 40 um | 200 - 400 HV, moderate | Barrier and decorative, 100 - 300 h | Conductive | Bright silver, takes a high polish | 100 percent build | 300 | 耐腐蚀防护 外观装饰 导电接地 | ASTM B689, ISO 1456 as a system | Barrier layer under gold, electronics hardware, decorative trim, solderable surfaces |
| Zinc plating, trivalent镀锌（三价钝化） | Plating, electrolytic | Steel | 5 - 25 um | 55 - 150 HV, cosmetic only | 96 h clear to 500 h yellow, lead free trivalent systems | Conductive, sacrificial to steel | Blue white, yellow, black | 100 percent build, thin enough to ignore on most features | 120 | 耐腐蚀防护 导电接地 | ISO 2081, ASTM B633, GB/T 9799 | Fasteners, brackets, welded and sheet metal frames, general hardware |
| Zinc nickel plating锌镍合金电镀 | Plating, electrolytic | Steel | 8 - 15 um | 200 - 280 HV | 720 - 1500 h salt spray, strongest of the zinc family | Conductive, sacrificial | Grey silver, black available | 100 percent build, mask fine pitch threads | 180 | 耐腐蚀防护 导电接地 | ISO 19598, ASTM B841, GB/T 5267.2 | Automotive fasteners, brake and under-hood hardware, high corrosion chassis parts |
| Zinc flake coating锌铝涂层（达克罗） | Dip and cure coating | Steel | 4 - 10 um | No hardening; works on class 12.9 without hydrogen embrittlement | 500 - 1000 h salt spray | Conductive, sacrificial | Silver grey or black, matte | 100 percent build but very thin, threads usually stay usable | 300 | 耐腐蚀防护 耐高温 | ISO 10683, GB/T 18684 | High strength fasteners, springs, chassis hardware, parts that cannot be baked after plating |
| Passivation, citric or nitric钝化 | Conversion coating | Stainless steel | Removes free iron, no layer added | No hardness change; it restores the passive film instead of adding a coating | Improves the alloy's own resistance by removing the contamination that starts pitting | Conductive, contact resistance unchanged | Unchanged to slightly brighter | No measurable size change, which is why it is safe on tight tolerances | 600 | 耐腐蚀防护 食品医药卫生 医疗器械 耐化学品 | ASTM A967, AMS 2700, GB/T 25150 | Every machined 300-series part bound for medical, food, pharma or chemical service |
| Electropolishing电解抛光 | Electrochemical removal | Stainless steel, some copper and aluminium | 5 - 25 um removed | No hardening; removes the damaged layer and deburrs in the same step | Improves: craters and inclusions go and chromium enriches at the surface | Conductive | Bright, mirror like, edges rounded | Removes 5 to 25 um per side, so finish before the final size or size the stock up | 600 | 食品医药卫生 医疗器械 耐化学品 低摩擦脱模 外观装饰 | ASTM B912, ISO 15730 | Hygienic pharma and food parts, medical instruments, semiconductor gas path components, fine deburring |
| Chromate conversion coating铬化处理（三价） | Conversion coating | Aluminum | 0.05 - 3 um | No hardening | Moderate alone, excellent as a paint base, 168 - 336 h | Keeps metal to metal contact: low contact resistance, which is why it is used for grounding and EMI | Clear to light yellow or blue | Effectively no size change | 120 | 导电接地 耐腐蚀防护 | MIL-DTL-5541, ISO 10546, GB/T 8013.2 | Chassis and enclosure grounding points, RF and EMI joints, paint and bond preparation |
| Black oxide发黑（黑氧化） | Conversion coating | Steel, stainless and copper variants | 0.5 - 2 um | No hardening | Minimal alone, 24 - 96 h, needs oil or wax to hold | Conductive | Matte black to deep black | About 1 um, negligible on most features | 200 | 外观装饰 耐腐蚀防护 耐磨 | MIL-DTL-13924, GB/T 15519 | Tooling, instrument components, low glare optical housings, parts that must look black |
| Phosphate coating磷化 | Conversion coating | Steel | 2 - 15 um | Porous crystalline layer, no hardening; manganese phosphate helps break-in | 24 - 100 h with oil, excellent paint and oil adhesion | Not used for grounding | Matte grey to dark grey | Small build, texture rather than thickness | 200 | 耐腐蚀防护 低摩擦脱模 外观装饰 | ISO 9717, GB/T 11376 | Gears and camshafts that must retain oil, base for paint, non reflective machine parts |
| Powder coating粉末喷涂 | Organic coating | Steel, aluminium | 60 - 120 um | Soft relative to metal; it protects, it does not carry load | 500 - 1000 h salt spray with proper pretreatment | Insulating | Any colour and gloss level, smooth or textured | The thickness dominates the tolerance: +60 to +120 um per side, mask threads and bores | 120 | 耐腐蚀防护 电绝缘 外观装饰 食品医药卫生 | Qualicoat and GSB systems, ISO 12944 system | Outdoor enclosures, frames, machine covers, consumer equipment; check the 180 to 200 C cure against aluminium temper |
| Liquid paint喷漆 | Organic coating | Steel, aluminium, plastics | 20 - 60 um | Soft | System dependent, 100 - 500 h | Insulating | Any colour, gloss or matte | Thinner than powder but still visible on a tolerance: +20 to +60 um per side | 120 | 耐腐蚀防护 外观装饰 电绝缘 | ISO 12944, SSPC systems | Large frames, welded assemblies, colour matching to a customer palette, touch-up |
| PTFE coating特氟龙涂层 | Dry film lubricant | Steel, aluminium, stainless with a primer | 15 - 40 um | Soft, friction coefficient 0.05 - 0.15, no load carrying hardness | Good chemical barrier | Insulating | Opaque white, dark green, grey or black by system | Build 15 to 40 um per side, mask sealing faces unless coating there is intended | 260 | 低摩擦脱模 耐化学品 食品医药卫生 | AMS 2515, coating supplier system specification | Mould release, pump and valve internals, food contact surfaces, low friction slides and seals |
| PVD coating, TiN TiAlN CrN DLC物理气相沉积 | Vapour deposition | Steel, stainless, carbide, titanium | 1 - 5 um | TiN 2000 - 2400 HV, TiAlN 3000 - 3400 HV, CrN 1600 - 2000 HV, DLC 2000 - 4000 HV with low friction | Depends on density: a porous coating on steel can accelerate galvanic attack, so pair it with passivation or nickel | TiN and CrN conductive, DLC semi insulating or insulating | TiN gold, TiAlN violet to black, CrN silver grey, DLC black | 1 to 5 um only, so a precision part stays a precision part | 600 | 耐磨 低摩擦脱模 外观装饰 耐高温 医疗器械 | ISO 14577 and EN 1071 test methods, coating supplier specification | Cutting tools, forming dies, mould surfaces, medical instruments, black wear parts that must stay dimensionally exact |
| CVD coating化学气相沉积 | Vapour deposition | Steel, carbide, ceramic | 5 - 20 um | 3000 - 4000 HV with excellent adhesion | Very good, dense layer | Conductive | Grey to black, matte | 5 to 20 um per side | 900 | 耐磨 耐高温 | ISO 14577, supplier specification | Indexable inserts, extrusion dies, nozzles; the 900 to 1000 C process means steel parts need a post-coating heat treatment |
| Bead blasting喷砂 | Mechanical surface | All metals, plastics with soft media | Removes 1 - 10 um | No hardening; shot peening is the process for that | Neutral: improves paint adhesion, but embedded fines hurt corrosion if not cleaned | Conductive | Uniform matte, satin, hides machining marks | Small removal, but sharp edges round and thin sections can move | n/a | 外观装饰 低摩擦脱模 前处理去毛刺 | ISO 8503 for the profile, SAE J444 for media | Satin finish before anodizing, weld and heat scale removal, mould cleaning, uniform appearance before coating |
| Shot peening喷丸强化 | Mechanical surface | Steel, aluminium, titanium | No material removed | Cold works the surface layer and raises fatigue life, two to ten times on a properly specified part | Neutral to slightly positive | Conductive | Matte, dimpled under a low angle light | Controlled by Almen intensity, not microns; it can move flatness, so peen before final grinding | n/a | 疲劳寿命 | SAE J442 and J443, AMS 2430 | Connecting rods, springs, gear roots, fillets that crack, landing gear and turbine hardware |
| Mechanical polishing机械抛光 | Mechanical surface | All metals | Removes to reach the target Ra | No hardening | Improves: coarse pits hold corrodents | Conductive | Satin to mirror | Removes enough to matter on a tight diameter, so polish before the final size | n/a | 外观装饰 低摩擦脱模 食品医药卫生 医疗器械 | ISO 21920 for the parameter, shop method | Optical surfaces, mould cavities, sealing faces, medical and food contact surfaces |
| Laser marking and engraving激光打标与雕刻 | Beam process | All metals, most plastics | 0.01 - 0.5 um marking | No change to the bulk material | Neutral; on stainless a light mark can slightly reduce local corrosion resistance | Conductive | Dark oxide mark, frosted, or colour on stainless | No measurable dimensional effect for marking, deep engraving removes real material | n/a | 标识追溯 外观装饰 | UDI per 21 CFR 830 for medical devices, customer marking specification | UDI and serial numbers, 2D Data Matrix codes, logos, scale marks, tool numbers |
| Nitriding, gas or plasma气体渗氮 / 离子渗氮 | Thermochemical diffusion | Alloy steels such as 42CrMo and 38CrMoAl, some stainless and titanium | Case 100 - 600 um deep | 900 - 1200 HV surface with a tough core: a diffusion layer, so it cannot chip off | Mild improvement | Conductive, surface resistance rises slightly | Grey to silver, with a visible size change on precision work | Growth of roughly 10 to 30 um on the surface, so grind datums and bores after treatment | 550 | 耐磨 表面硬化 耐高温 疲劳寿命 耐腐蚀防护 | GB/T 18177, AMS 2759 series | Gear and spline flanks, mould and die surfaces, pump and valve internals, wear critical shafts. Low distortion because there is no quench |
| Carburizing and hardening渗碳淬火 | Thermochemical diffusion | Low carbon steels such as 20CrMnTi and 8620 | Case 300 - 2000 um deep | 58 - 62 HRC case with a tough core; the classic gear treatment | Poor alone, needs plating or paint | Conductive | Grey to dark grey after tempering | Quench distortion is real, so leave grinding stock on datums | 200 | 耐磨 表面硬化 疲劳寿命 | GB/T 9450, ISO 2639 for case depth | Gears, pinions, cams, splines, bearing races, parts needing a hard skin and a shock resistant core |
| Induction hardening感应淬火 | Local heat treatment | Medium carbon and alloy steels such as 1045 and 4140 | Case 1000 - 5000 um deep | 55 - 62 HRC in the treated band only | Poor in the treated zone without protection | Conductive | Dark martensitic grey | Distortion limited to the quenched zone, finish grind after treatment | 250 | 耐磨 表面硬化 疲劳寿命 | GB/T 5617 | Shaft journals, splines, gear teeth, bearing seats, the local wear band on a large part |
| Nitrocarburizing, salt bath or gas氮碳共渗（QPQ 类） | Thermochemical diffusion | Steel, stainless | Compound zone 10 - 30 um plus diffusion | 600 - 800 HV with an oxidised black outer layer that also cuts friction | 200 - 500 h with the oxidation step, very good for a black finish | Conductive | Uniform matte black | Same order of growth as nitriding: allow 10 to 30 um | 400 | 耐磨 耐腐蚀防护 外观装饰 低摩擦脱模 | No single ISO number: specify compound zone and diffusion depth on the drawing | Black wear parts, hydraulic components, fasteners, parts that need wear plus black appearance in one step |
| Titanium anodizing钛阳极氧化 | Anodizing | Titanium and titanium alloys | 0.02 - 1 um | Thin oxide that greatly reduces galling and fretting | Titanium is already passive: this is for function and identification, not extra corrosion | Effectively conductive at low voltage, very thin dielectric layer | Interference colours by voltage: bronze, purple, blue, gold, green | Under 0.2 um, so precision is preserved | 300 | 医疗器械 外观装饰 低摩擦脱模 耐腐蚀防护 | ASTM F86, AMS 2488 | Surgical instruments, implant identification marking, titanium fasteners, parts that must not gall |
Red flags worth catching at drawing stage: a tight bore that will be anodized or plated (compensate or mask), a 0.005 mm flatness callout on a part that will be powder coated (the coating is thicker than the tolerance), a grounding stud on an anodized chassis, a hard anodized fillet that must survive fatigue, and a PVD finish on a rough surface, where the film follows every peak.
Surface texture: Ra and Rz
Value series per GB/T 1031-2009 (ISO 4287). Ra is the default parameter; Rz controls peak-to-valley height where Ra cannot, on very fine or very rough surfaces. Prefer the lower end only where function demands it: tighter Ra costs cycles and can hurt lubricant retention.
Ra series (um)
0.012 0.025 0.05 0.1 0.2 0.4 0.8 1.6 3.2 6.3 12.5 25 50 100
Rz series (um)
0.025 0.05 0.1 0.2 0.4 0.8 1.6 3.2 6.3 12.5 25 50 100 200 400 800 1600
Sampling lengths
| Ra band (um) | Sampling length lr (mm) | Evaluation length ln (mm) |
|---|---|---|
| >=0.008 to 0.02 | 0.08 | 0.4 |
| >0.02 to 0.1 | 0.25 | 1.25 |
| >0.1 to 2.0 | 0.8 | 4 |
| >2.0 to 10.0 | 2.5 | 12.5 |
| >10.0 to 80.0 | 8 | 40 |
Where each Ra is used
| Ra (um) | Typical application (GB/T 1031 condensed) |
|---|---|
| 0.8 | Fits needing guaranteed concentricity: P6 and P5 rolling bearing seats, spigot bores IT7, plain pins IT8 |
| 0.2 | Long life mating surfaces: IT6 and IT5 bores and shafts, gear teeth 6th grade, journal bearings P5 |
| 0.1 | Heavily loaded, variable stress surfaces: hydraulic cylinder bores and rods, gland grooves |
| 0.05 | Spindle bearings P4 and above, friction faces of clutches, precision gauge measuring faces |
| 0.025 | Precision gauge balls and rolls, high pressure pump plunger pairs, gauge blocks over 100 mm |
Rough rule of thumb: Rz is roughly 4 to 7 times Ra for turned and milled surfaces. Do not convert for acceptance: measure the parameter the drawing specifies.
What each machining process can hold
Use this to sanity check a finish callout against the process route. If the drawing asks for Ra 0.2 on a milled pocket, the route has to change to grinding, lapping or polishing, and the price changes with it.
| Machining process | Achievable Ra (um) |
|---|---|
| Rough milling | Ra 3.2 - 12.5 um |
| Finish milling | Ra 0.8 - 1.6 um |
| Fine milling, high speed small stepover | Ra 0.4 - 0.8 um |
| Rough turning | Ra 3.2 - 6.3 um |
| Finish turning | Ra 0.8 - 1.6 um |
| Fine turning with wiper insert | Ra 0.4 - 0.8 um |
| Drilling | Ra 3.2 - 12.5 um |
| Reaming | Ra 0.8 - 1.6 um |
| Fine boring | Ra 0.4 - 0.8 um |
| Broaching | Ra 0.4 - 1.6 um |
| Surface grinding | Ra 0.1 - 0.8 um |
| Cylindrical grinding | Ra 0.1 - 0.4 um |
| Honing | Ra 0.05 - 0.4 um |
| Lapping | Ra 0.012 - 0.1 um |
| Wire EDM, multi pass | Ra 0.2 - 1.6 um |
| Die sinking EDM | Ra 0.4 - 3.2 um |
| Bead blasting | Ra 1.6 - 6.3 um |
| Shot peening | Ra 1.6 - 6.3 um |
| Mechanical polishing | Ra 0.012 - 0.1 um |
| Electropolishing | Ra 0.05 - 0.4 um |
| Decision rule | Practical effect |
|---|---|
| Ra 3.2 or coarser | As machined. No extra operation, no extra cost. |
| Ra 1.6 | Normal finish pass. Default for most mating and sealing faces. |
| Ra 0.8 | Fine pass with a sharp tool and a stable setup. Adds inspection, not usually cost. |
| Ra 0.4 or finer | Adds a finishing operation: grinding, honing, lapping or polishing. Price step here. |
| Ra 0.1 or finer | Specialist territory. Specify the parameter and the measuring method, and expect a dedicated route. |
Finish fights heat treatment and coating: nitriding roughens a ground face, hard anodize follows every machining mark, and blasting sets a floor of about Ra 1.6 that polishing must then remove.
How KGL specifies and verifies finish
| What we do | Why it matters to your price |
|---|---|
| Finish stated as a parameter plus a value on the drawing | Ra 0.8 with the measuring method noted, so inspection and machining agree on the acceptance rule |
| Tolerance and coating checked together before quoting | If the coating is thicker than the tolerance, we say so at quotation stage rather than after plating |
| Batch anodizing and coating for one order | Same alloy, same bath, same batch keeps colour and thickness consistent across the shipment |
| Treatment before final datum grinding | Nitrided and hardened parts are ground after treatment, so the reference geometry is real |
| Surface verification on the CMM plus a roughness check | Profile tolerance, Ra and layer thickness all reported on the same inspection sheet where the drawing calls for them |
| Question we hear | Short answer |
|---|---|
| Can I anodize and still hold the bore? | Yes, if we compensate or mask. Tell us which faces must stay exact and we quote the masking. |
| Why did my black anodize come out brown? | Alloy chemistry, dye bath condition and layer thickness all shift shade. Batch coating and a retained sample keep it repeatable. |
| Chem film or anodize for a grounded enclosure? | Chem film, because it keeps metal to metal contact. Anodize gives better wear and colour but insulates. |
| Best wear finish on aluminium? | Hard anodize, or PVD if the geometry must not change. On steel, nitriding holds far higher load. |
Where these finishes are produced and checked
Ra and Rz come out of the process, not out of the drawing. These pages show which operation produces the band, what the treatment adds and how the result is verified.
Operations that produce the finish
- CNC millingFacing, contouring and finishing passes that set the as-machined Ra.
- CNC turningTurning inserts, feed per revolution and the Ra they leave behind.
- Surface and cylindrical grindingWhere the drawing asks for a band that cutting alone cannot hold.
Treatments applied on top
- Surface treatmentAnodising, passivation, plating, PVD and heat treatment, with size change per process.
- Inspection and QCRa and Rz measured on the finished surface, not assumed from the process.
Where the finish is a functional requirement
- Medical devicesSurfaces that must be cleanable and documented under ISO 13485.
- Semiconductor equipmentSealing faces and vacuum-side surfaces where Ra drives leak rate.
- AerospaceFinish and coating call-outs released under AS9100D.
Have a feature that sits at the edge of one of these bands? Send the drawing and the tolerance question with the drawing and we will answer against it.
Finish and tolerance fighting each other on your drawing?
Send the 2D and 3D files. We return a quote in 24 hours and flag the places where the coating, the tolerance and the material disagree, before the parts are cut. No MOQ, single prototypes supported.