VDI 3198 — Quality Assurance of PVD and CVD Coatings

TLDR

VDI 3198 (2017) is the VDI standard for quality assurance of PVD and CVD coatings on metallic substrates. It defines the HF1–HF6 acceptance categories based on three complementary adhesion tests (Rockwell indentation, scratch-test critical load, and impact / bending) and is the most widely cited European acceptance framework for PVD coatings on titanium, tool, and automotive parts.

Quick Answer

What does VDI 3198 cover?
VDI 3198 establishes a six-tier acceptance classification (HF1 best, HF6 worst) for PVD / CVD coatings based on:

1. Rockwell indentation (HRC). Visual inspection of the indentation crater for cracking or delamination.
2. Scratch test (per ASTM C1624 or equivalent). Critical load Lc at which the coating fails by cohesive or adhesive spallation.
3. Impact / bending fatigue. Coating survival under cyclic loading.

The categories map onto critical-load values from ASTM C633 bond-strength testing and onto ASTM B487 thickness measurement for full acceptance documentation.

Scope

VDI 3198 applies to PVD and CVD coatings on metallic substrates including titanium, tool steel, carbide, and stainless steel. For titanium PVD (TiN, TiAlN, DLC), it is the European acceptance standard for automotive, tooling, and decorative applications. North-American aerospace programs use AMS 2444 + ASTM C633 but cite VDI 3198 for category mapping.

HF category overview:

Category	Indentation (HRC)	Scratch (Lc, N)	Typical Application
HF1	No cracks, no delamination	≥ 60	Premium cutting tools, medical implants
HF2	Micro-cracks, no delamination	45–60	Aerospace fasteners, high-end tooling
HF3	Cracks, minor delamination	30–45	Automotive, consumer hardware
HF4	Delamination around indentation	20–30	Decorative consumer
HF5	Significant delamination	10–20	(rarely acceptable)
HF6	Coating failure	< 10	Rejected

Test Methods

1. Rockwell indentation (HRC). Apply a standard Rockwell-C indentation at 1471 N load; inspect the crater under 100× optical microscopy for radial cracks, circumferential cracks, and coating delamination. The indentation produces a defined stress field; the coating’s response is a direct indicator of adhesion.
2. Scratch test. Apply a linearly-increasing normal load through a diamond stylus (typically 200 µm radius) while translating at constant velocity (typically 10 mm/min over 10 mm). Detect the critical load Lc by acoustic emission, friction-force change, and optical inspection of the scratch track. Three critical-load values are recorded: Lc1 (first cohesive cracking), Lc2 (first adhesive spallation), Lc3 (full delamination).

Process Mapping

The VDI 3198 categories map onto process parameters as follows:

VDI Category	Substrate Bias	Deposition T	Adhesion (ASTM C633, N)	Pre-coat Ra
HF1	−80 to −150 V	350–450 °C	≥ 60	0.2–0.4 µm
HF2	−50 to −100 V	300–400 °C	45–60	0.2–0.5 µm
HF3	−30 to −80 V	200–350 °C	30–45	0.2–0.8 µm
HF4	−30 to −50 V	150–300 °C	20–30	0.4–1.0 µm

These are typical mappings; the OEM specification should be the primary acceptance reference.

Common Pitfalls

• HF category drift between lots. Without reference coupons and SPC, the HF category can drift from HF2 to HF4 across a production run. Use a statistically-significant number of test coupons per lot.
• Indentation load sensitivity. The Rockwell-C 1471 N load is standardized; lower loads (HRB, HR30N) give more lenient HF categories and should not be substituted.
• Scratch-test stylus wear. A worn diamond stylus gives artificially-low Lc values; inspect the stylus tip before each test series.
• Substrate mismatch. The HF category depends on the substrate; HF1 on carbide is not equivalent to HF1 on Grade 5 titanium. Report the substrate with the category.

Related Standards

• AMS 2444 — Coating, Physical Vapor Deposition of Titanium Nitride. Aerospace process specification for TiN PVD on titanium; references VDI 3198 for European acceptance.
• ASTM C633 — Adhesion or Cohesion Strength of Thermal Spray Coatings. Tensile-bond test that complements VDI 3198 indentation and scratch tests.
• ASTM B487 — Microscopical Cross-Section Thickness Measurement. Thickness verification; HF category is thickness-dependent.
• ISO 14577 — Instrumented Indentation Testing. Nanoindentation method for coating hardness; complements VDI 3198 for mechanical-property characterization.
• AMS 2700 — Passivation of Corrosion-Resistant Steels and Titanium Alloys. Pre-PVD cleaning sequence affecting HF category.

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. 3. Impact / bending. Cyclic impact or three-point-bending fatigue to verify the coating survives mechanical loading expected in service. Less standardized than the indentation and scratch tests; OEM-specific.

HF1–HF6 Acceptance Categories

The HF categories combine the three test results into a single acceptance label:

• HF1 (best). No cracks, no delamination at HRC indentation; Lc2 ≥ 60 N. Reserved for premium cutting tools, high-stress medical implants, and aerospace primary structure.
• HF2. Micro-cracks at HRC indentation, no delamination; Lc2 = 45–60 N. Typical for aerospace fasteners and high-end tooling.
• HF3. Visible cracks, minor delamination; Lc2 = 30–45 N. Typical for automotive wear surfaces and consumer hardware.
• HF4. Delamination around indentation; Lc2 = 20–30 N. Decorative consumer; functional hardware with light wear.
• HF5. Significant delamination; Lc2 = 10–20 N. Rarely acceptable; engineering judgment required.
• HF6 (worst). Coating fails at indentation; Lc2 < 10 N. Rejected; coating strip and re-coat.

For titanium PVD, the typical aerospace acceptance is HF1–HF2 (TiN, TiAlN on Grade 5 fasteners and hydraulic components); medical implants typically accept HF1–HF3; consumer hardware accepts HF3–HF4.