Tool Steel Coatings: Enhancing Surface Properties and Wear Resistance

Introduction

Tool steels are extensively utilized for manufacturing cutting, forming and molding tools due to their hardness, strength and other properties. However, even tool steel components are susceptible to wear, erosion, corrosion and other degradation mechanisms during service.

Applying protective coatings or surface treatments provides an effective means of enhancing the surface-related properties of tool steels to prolong service life. This article explores the wide range of coating technologies available and provides guidance on selecting appropriate coatings to combat specific wear, friction, corrosion and other performance issues in various tooling applications.

Overview of Tool Steel Coating Technologies

A variety of coating types can protect tool steels:

PVD coatings – TiN, TiCN, TiAlN, CrN, ZrN, multilayers
CVD coatings – TiC, TiCN, TiN, Al2O3
DLC coatings – Diamond-like carbon, amorphous carbon, hydrocarbon polymers
Nitriding/carburizing – Diffusion treatments for surface hardening
Electroplating – Chrome, nickel, copper, tin & alloys
Thermal sprays – Combustion or electric arc spraying of carbides, metals, alloys
Enamels – Vitreous coatings fused onto steel surface

Selecting the optimal coating depends on service conditions, cost, performance needs, and other factors.

Benefits and Limitations of Tool Steel Coatings

Potential advantages of coatings:

Increased surface hardness to resist wear and abrasion
Create lubricious surface to minimize friction and galling
Provide chemical/corrosion protection from liquids or environments
Improve resistance to metal adhesion during forming or cutting
Can repair worn surfaces and restore dimensions
Allow lower cost substrate with coating only where needed

Limitations can include adhesion failures, insufficient thickness, porous coatings, or inadequate quality control during application.

Key Tool Steel Coating Properties

Critical coating performance criteria:

Hardness – Ability to resist cutting, indentation, abrasive wear
Adhesion – Bond strength between coating and substrate
Coefficient of Friction – Ability to minimize friction and galling
Oxidation Resistance – Stability and protection at elevated temperatures
Corrosion Resistance – Protection against liquids, chemistries, environments
Fatigue Strength – Ability to resist repeated cycling without cracking
Coating Stresses – Compressive stresses enhance performance; tensile stresses detrimental
Coating Thickness – Sufficient thickness needed for protection in service conditions

The desired combination of coating properties depends on the operating environment and tooling application specifics.

PVD Coatings for Tool Steels

PVD (physical vapor deposition) applies thin (2-5 μm), hard coatings via vaporization and deposition processes:

TiN – Benchmark gold coating offers moderate hardness, good lubricity and oxidation resistance
TiCN – Controls friction well with improved wear resistance over TiN
TiAlN – Excellent hot hardness and oxidation resistance at 800°C
CrN – Hard chromium nitride coating with good corrosion protection
ZrN – Extremely low friction coefficient; suitable for aluminum forming
Multilayer – Alternating nanolayers of TiN and TiCN or other materials

PVD excels at smooth, uniform, dense coatings applied at lower temperatures ideal for sharp cutting tools and forming dies.

CVD Coatings for Tool Steels

CVD (chemical vapor deposition) coatings involve chemical reactions on heated surfaces:

TiC – Extreme abrasion resistance; brittle
TiCN – Good oxidation and wear resistance
Alumina – Low friction, temperature resistant ceramic
Diamond – Unmatched wear performance but high cost

CVD can deposit thicker coatings than PVD and excels at coating complex cutting tool geometries but has higher processing temperatures.

DLC Coatings for Tool Steels

Diamond-like carbon (DLC) coatings offer exceptional properties:

Extreme hardness approaching diamond
Very low coefficient of friction
Chemical inertness for corrosion protection
Smooth finish for low adhesion

Drawbacks are internal stresses can cause adhesion failures. DLC excels at low-mid temperature applications of cutting tools, dies and molds.

Electroplated Coatings for Tool Steels

Electroplating provides adherent metallic coatings:

Hard Chrome – Thick chromium for wear and corrosion protection
Nickel – Corrosion and wear resistance with lubricity
Copper – Low cost coating for mild corrosion environments
Tin – Lubricity and release properties

Electroless nickel platings offer uniformity on complex geometries. Electroplating provides very economical but porous coatings.

Thermal Spray Coatings

Thermal spray uses compressed gas flame or electric arc to melt-spray micron sized particles onto surfaces:

WC-Co – Extreme wear resistance; thermal shock issues
Cr3C2-NiCr – Excellent abrasion resistance and toughness
Aluminum Bronze – Anti-galling; embedded lubricants

Thermal spray excels at thicker, inexpensive coatings on large components but has poorer adhesion than PVD/CVD.

Nitriding and Carburizing of Tool Steels

Nitriding/carburizing diffuses nitrogen/carbon into surface for a hard case:

Gas nitriding – Ammonia gas provides simple low cost surface hardening
Plasma nitriding – Cleaner, lower temperature process
Laser nitriding – Solid state surface enhancement; minimal distortion
Gas/plasma/laser carburizing – Diffusion of carbon for extreme surface hardness

Ideal for selective hardening of local areas on large tools/dies where coating impractical.

Coating Process and Quality Considerations

Critical process and quality factors:

Surface preparation to ensure adhesion – cleaning, blasting, etching
Process control and validation to ensure uniformity and repeatability
Post-coating heat treatments or stress relief
Coating property verification – hardness, thickness, adhesion testing
Dimensional conformance checks post-coating
Surface inspection – verify coating integrity; minimize defects
Sample testing under expected use conditions

Rigorous quality methods and testing validate coating performance.

Matching Tool Steel Coatings to Applications

Proper coating selections for sample applications:

Punches and forming dies – Hard lubricious coatings (TiN, TiCN, CrN) resist galling and transfer
Cutting tools – Low friction, temperature resistant coatings (TiAlN, AlTiN) for high speeds
Molds – Thick, corrosion resistant chrome coatings protect mold surfaces
Gauges, fixtures – Thin dense chrome for micro-abrasion and corrosion protection
Blanks, slitters – Low-friction ZrN, W-DLC helps prevent work material adhesion
Extrusion/drawing dies – Multilayer nano-structured coatings maintain surface finish

Considering the service environment and failure modes is key to optimal coating selection.

Coating Innovations for Next Generation Tool Steels

Ongoing coating developments enable performance improvements:

Multilayer architectures with nanoscale modulated chemistry
Multicomponent and multifunctional coatings
Graded and composite coatings to transition properties
Superlattice structures for enhanced hardness and toughness
New precursors and green coatings based on environmentally friendly materials
Advanced direct vapor deposition techniques like arc-vapor deposition
Coatings engineered specifically for additive manufactured tool steels

Exciting innovations continue advancing coating technologies and capabilities.

Summary

A wide range of coating technologies can protect tool steel surfaces
Matching coating properties to service conditions ensures optimal performance
PVD, CVD and DLC excel for thin hard coatings; Thermal spray for thicker coatings
Nitriding/carburizing provides localized hardness and wear resistance
Coating process control and inspection ensures coating quality
New coating developments expand possibilities for tool steel enhancement

Applying appropriate protective coatings to tool steels prolongs service lifetimes, enhances performance, and reduces maintenance costs.

Frequently Asked Questions

What are some key benefits of coatings for tool ocels?

Coatings protect against wear, galling, and corrosion. They can restore dimensions of worn components and enable use of cheaper substrate materials. Coatings also facilitate easy repair of tooling by re-coating worn areas.

When should coatings vs. heat treating or bulk alloy selection be used?

Coatings serve as supplemental protection when the base alloy can’t provide adequate wear resistance or corrosion protection by itself. However, coatings complement but don’t replace proper heat treating and bulk alloy selection.

What causes coating adhesion failures?

Poor surface preparation, residual stresses, coating process defects, thermal expansion mismatch between coating and substrate, and third body abrasion attacking the coating-substrate interface can all cause adhesion failures.

How thick can modern PVD coatings be?

PVD is typically limited to 2-5 μm coating thickness. Beyond this can result in high residual stresses, adhesion failures, and reduced performance. CVD and thermal spray produce much thicker coatings.

What innovations show good potential for next generation tool steel coatings?

Multilayer architectures with nanoscale layers, new multifunctional compositions, advanced direct vapor deposition techniques, and coatings tailored specifically for additive tool steels.

Please let me know if you have any other questions!