2. Materials and Methods
2.2. Heat Treatment and Coatings
2.3. Mechanical Properties
2.4. Load-Carrying Capacity and Wear Testing
3.1. Tool Steel Substrate Properties Correlation
3.2. Thermo-Chemical Treatment vs. Coating
3.3. Effect of Substrate Hardness and Fracture Toughness on Coating Performance
3.4. Effect of Substrate Roughness
- Yield and maximum compression and bending strength of the tool steel substrate show rising linear dependency on hardness and reduced trend with fracture toughness. On the other hand, strain hardening exponent has no direct correlation with hardness but shows rising trend with increased fracture toughness.
- Thermo-chemical treatment, i.e., plasma nitriding, provides up to 25% better tool steel wear resistance. However, even when plasma nitrided wear resistance depends on combination of fracture toughness and core hardness. Higher hardness of the core material improves abrasive wear resistance of the surface, but sufficient fracture toughness level needs to be provided. On the other hand, hard wear-resistant coatings outperform all other surface engineering techniques, providing up to two orders of magnitude better tool abrasive wear resistance.
- In the case of coated applications steel substrate must provide sufficient load-carrying capacity and support for the coating. A compound layer can be used as an additional interlayer, but its brittleness results in accelerated coating cracking as the core hardness is reduced. Even for cases without compound layer, high level of steel core hardness (above 50 HRC) is mandatory in order to provide good load support for the top coating. However, high hardness must also be supported by proper level of the fracture toughness (above 30 MPa√m).
- Surface roughness and topography have major influence on galling resistance in forming, with smoother surfaces and plateau-like topography providing better results. This is further escalated for coated surfaces, where galling resistance depends on substrate roughness level, coating type and material to be formed. In the case of typical hard ceramic coatings post-polishing of the coated surface and use of smoothened substrate gives about 2 times better galling resistance. On the other hand, for carbon-based low friction coatings post-polishing and roughness of the substrate have very limited effect on the tool resistance against galling and work material transfer.
- Abrasive wear resistance as well as surface quality of hot work tool steel mainly depend on hardness but also on fracture toughness. Good machinability and the best surface quality are obtained when hardness is between 45 and 50 HRC, and high abrasive wear resistance for hardness above 48 HRC. Fracture toughness, however, should be below 55–60 MPa√m to get best performance. On the other hand, coefficient of friction is independent on heat treatment parameters and mainly depends on contact conditions.
- In the case of cold work tool steel, hardness of over 64 HRC is required to obtain sufficient load-carrying capacity of the coated surface, regardless of the coating type used. However, required level of the fracture toughness is dependent also on the coating type. In the case of monolayer coatings, the main parameter is hardness of the substrate with higher the better. For typical multilayer coatings, having improved resistance to crack initiation and propagation, substrate hardness of about 64 HRC is sufficient and high fracture toughness only required at low hardness. However, for very brittle coatings combination of working hardness and high fracture toughness (above 10 MPa√m) gives superior results.
- Substrate hardness is the most influential parameter also when it comes to impact wear resistance, which is true for different types of hard coatings. However, except for very brittle coatings fracture toughness of the steel substrate should be above 12 MPa√m and hardness in the range of 64–65 HRC, thus providing combination of high load-carrying capacity, good fatigue properties and superior resistance against impact wear.
Conflicts of Interest
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|No.||Tool Steel||% C||% Si||% Mn||% Cr||% Mo||% V||% W||% Co||% Fe|
|Temp. [°C]||Time [min]||Temp. [°C]||Immersion Time [h]||Temp. [°C]||Time [h]|
|Substrate Treatment||Fracture Toughness KIc [MPa√m]||Hardness HRC||KIc/HRC|
|A1||6.1 ± 1.2||65.8 ± 0.2||0.09|
|A2||10.2 ± 2.0||64.0 ± 0.2||0.159|
|A3||12.7 ± 0.7||59.3 ± 0.1||0.214|
|B1||10.4 ± 0.8||65.0 ± 0.3||0.160|
|B2||12.4 ± 0.7||64.2 ± 0.4||0.193|
|B3||14.2 ± 0.4||59.5 ± 0.1||0.239|
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