ASTM C633 — Adhesion or Cohesion Strength of Thermal Spray Coatings
Engineering reference for ASTM C633-13, the standard tensile-bond test method for adhesion or cohesion strength of thermal-spray and PVD coatings. Covers test specimen preparation, fixture design, tensile loading procedure, and acceptance criteria for ceramic and metallic coatings on metal substrates including titanium.
TLDR
ASTM C633-13 is the ASTM International standard test method for determining the adhesion or cohesion strength of thermal-spray and physical-vapour-deposited coatings by tensile loading. It is the de-facto bond-strength test for PVD (TiN, TiAlN, DLC) coatings on titanium substrates, with functional aerospace and medical acceptance typically in the 30–70 N critical-load range.
Quick Answer
What does ASTM C633 cover?
ASTM C633 specifies the test-specimen geometry, fixture design, loading-fixture adhesive, tensile loading procedure, and calculation of adhesion (or cohesion) strength for ceramic and metallic coatings on metal substrates. The method was developed for thermal-spray coatings but is widely adapted to PVD coatings on titanium. Acceptance for functional PVD on titanium is typically ≥ 30 N (HF1–HF4 per VDI 3198).
Scope
ASTM C633 applies to coatings applied by thermal-spray, physical-vapour-deposition (PVD), and chemical-vapour-deposition (CVD) processes on metallic substrates. For titanium PVD, the method is the workhorse bond-strength test referenced by AMS 2444, VDI 3198, and most aerospace / medical OEM specifications.
Key test parameters:
- Specimen diameter: 25.4 mm (1 in) standard cylindrical button
- Loading rate: 1.0 mm/min (crosshead displacement)
- Failure modes: adhesive (coating/substrate), cohesive (within coating), adhesive (coating/adhesive)
- Acceptance (TiN PVD on titanium): 30–70 N critical load for HF1–HF4 acceptance
Test Procedure
- Mount. Align substrate and coupling buttons in the tensile fixture; verify concentric loading.
- Pre-load. Apply 10 N preload to seat the assembly.
- Load. Pull at 1.0 mm/min crosshead displacement until failure; record peak load (N) and failure mode.
- Inspect. Document fracture surface — classify failure as adhesive (coating/substrate interface), cohesive (within coating), or adhesive failure at the epoxy/coupling interface (invalid test).
- Report. Report peak load (N), calculated stress (MPa = load / area), failure mode, and any visible defects.
Calculations
The bond strength is calculated as:
σ = F / Awhere:
- σ = adhesion or cohesion strength (MPa)
- F = peak tensile load at failure (N)
- A = cross-sectional area of the test button (mm²), typically 506.7 mm² for the 25.4 mm diameter
For a 25.4 mm button, peak loads of 30–70 N correspond to bond strengths of approximately 0.06–0.14 MPa — these low values are characteristic of PVD and are typically reported as critical load (N) rather than stress (MPa).
Acceptance Criteria
The acceptance window depends on the application and the OEM specification. Typical functional acceptance windows:
| Application | Critical Load (N) | VDI 3198 Category | Notes |
|---|---|---|---|
| Decorative consumer | ≥ 15 | HF4–HF6 | Gold color, low wear |
| Aerospace fastener (TiN) | ≥ 30 | HF1–HF4 | Functional wear / identification |
| Medical implant (TiN) | ≥ 30 | HF1–HF3 | Biocompatibility + wear |
| Cutting tool (TiAlN) | ≥ 50 | HF1–HF2 | High-stress service |
| Optical DLC | ≥ 30 | HF1–HF2 | Adhesion critical for low-friction |
Common Pitfalls
- Epoxy failure (invalid test). If the epoxy bond at the coupling interface fails before the coating, the test is invalid; the epoxy tensile strength must exceed the expected coating bond strength.
- Substrate failure (invalid test). For thin substrates or low-strength grades (e.g., Grade 1) the substrate itself can yield before the coating fails; use a thicker substrate or a higher-strength grade for the test coupon.
- Off-center loading. Misaligned fixtures create bending stress that lowers the apparent bond strength.
- Inconsistent surface prep. Test coupons must be prepared with the same surface prep as the production part; a “polished lab coupon” gives higher bond strength than a “production coupon” and is not representative.
- Single-test reporting. Bond strength is process-dependent; report a minimum of 5 coupons per lot and use the mean ± standard deviation.
Related Standards
- AMS 2444 — Coating, Physical Vapor Deposition of Titanium Nitride. Aerospace process specification for TiN PVD on titanium; ASTM C633 is the bond-strength test method.
- VDI 3198 — Quality Assurance of PVD and CVD Coatings. European acceptance categories HF1–HF6; maps onto ASTM C633 critical loads.
- ASTM B487 — Microscopical Cross-Section Thickness Measurement. Companion test method for coating thickness verification.
- ISO 14577 — Instrumented Indentation Testing. Nanoindentation method for coating hardness; complements C633 for coating mechanical-property characterization.
- AMS 2700 — Passivation of Corrosion-Resistant Steels and Titanium Alloys. Pre-PVD cleaning sequence affecting C633 result.
For an overview of where PVD fits in the broader titanium surface-finish landscape, see the surface finishes hub and the dedicated PVD coating guide.
Test Specimens
- Substrate button. Cylindrical substrate, 25.4 mm diameter × 25.4 mm length; substrate material must match production part (e.g., Grade 2 or Grade 5 titanium per the application).
- Coupling button. Identical geometry to substrate; bonded to the coated face of the substrate.
- Surface preparation. Substrate face is prepared per the production process — pre-clean, mask, and coat as if it were a production part.
- Coupling adhesive. High-strength epoxy (typically FM 1000 or equivalent) cured per adhesive manufacturer’s specification; adhesive tensile strength must exceed the expected coating bond strength.