cutting tool technology - tool coating

New workpiece materials are being developed in response to many factors. These include ever-increasing global demand for high performance components, and higher safety standards. Development of these new alloys has always been one of the main drivers of new cutting tool technologies and grades.

Improved properties that allow parts made from these materials to perform better in their intended application also present special challenges for milling tools. Some of the key properties required for milling tools to machine these high-performance work materials include:
~ High hot hardness, i.e., retention of the cutting edge at elevated temperatures near the tool/workpiece interface.
~ Ability to withstand high cutting forces during machining.
~ Low thermal conductivity to resist edge degradation such as depth-of-cut notching, plastic deformation, and oxidation caused by high temperatures at the cutting edge.
~ Chemical inertness to minimize formation of built-up edge (BUE) and the possibility of coating delamination.
~ High wear resistance to reduce abrasive wear at the cutting edge due to hard intermetallic compounds in the microstructure.
~ Geometry that provides efficient cutting, good chip-breaking, and minimizes heat generation during machining to reduce subsurface defects on the workpiece.


Coating, is one favored method used by many toolmakers. By coating, one can gain two material characteristic, one obtained from the core material, and another from the layer of coating. Here listed several characteristic of coating :


HARDNESS
A high surface hardness from your coating is one of the best ways to increase tool life. Generally speaking, the harder the material or surface, the longer the tool will last. Titanium Carbo-Nitride (TiCN) has a higher surface hardness versus that of Titanium Nitride (TiN). The addition of carbon gives TiCN 33 percent higher hardness and changes the range from about 3,000 to 4,000 Vickers, depending on the manufacturer. With a surface hardness near 9,000 Vickers, CVD diamond coatings that have been grown onto the tools show 10 to 20 times better tool life compared to PVD coatings. This is the coating of choice for production work in non-ferrous materials because of its higher hardness and its ability to run at two to three times the speed of uncoated tooling.

WEAR RESISTANCE
This is the ability of the coating to protect against abrasion. Although a material may not be hard, elements and processes added during production may aid in the breakdown of cutting edges or forming lobes.

SURFACE LUBRICITY
A high coefficient of friction causes increased heat, leading to a shorter coating life or coating failure. However, a lower coefficient of friction can greatly increase tool life. The amount of heat can be reduced by a surface that lacks coarseness or irregularities. This slick surface lets the chips slide off the face of the tool, generating less heat. A higher surface lubricity also can allow for increased speeds when compared to non-coated versions. This further wards off galling of the work material.

OXIDATION TEMPERATURE
This is the point at which the treatment starts to break down. A higher oxidation temperature rating improves success in high heat applications. Although the Titanium Aluminum Nitride (TiAlN) coatings may not be as hard as TiCN at room temperature, it proves to be much more effective in applications where heat is generated. This coating holds its hardness at higher temperatures due to a layer of aluminum oxide that forms between the tool and the cutting chip. This layer transfers heat away from the tool and into the part or chip. Carbide tooling is generally run at higher speeds compared to HSS. This makes TiAlN a preferred choice when coating carbide. Drills and endmills are commonly coated with this type of PVD treatment.

ANTI-SEIZURE
This property keeps material from depositing onto the tool by preventing less chemical reactivity between the tool and the cutting material. BUE (Built Up Edge) is very common in non-ferrous materials like aluminum or brass. BUE can lead to chipping of the tool or oversizing of the part. Once the material starts adhering to the tool, it continues to attract. In the case of machining aluminum with a forming tap, aluminum deposits grow larger after every hole. Eventually, the pitch diameter becomes so enlarged that the part becomes oversized and needs to be scrapped. A coating with increased anti-seizure properties may even be able to aid where poor coolant quality or concentration is a problem.



Here are several common coating materials and its characteristics :
• Titanium Nitride (TiN)
General purpose PVD coating that increases hardness and has a high oxidation temperature. This coating works great while cutting or forming with HSS tooling.
• Titanium Carbo-Nitride (TiCN)
The addition of carbon adds more hardness and better surface lubricity. This coating is ideal for HSS cutting tools.
• Titanium Aluminum Nitride (TiAlN or AlTiN)
A formed layer of aluminum oxide gives this tool better life in high heat applications. This coating is primarily selected for carbide tooling where little to no coolant is being used. AlTiN offers a higher surface hardness than that of TiAlN, along with different percentages of aluminum and titanium. It is another viable option in the world of HSM.
• Chromium Nitride (CrN)
The anti-seizure properties of this coating makes it preferred in situations where BUE is common. HSS or carbide cutting and forming tools will be seen with this almost invisible coating.
• Diamond
A CVD process that offers the highest performance available in non-ferrous materials. Ideal for cutting graphite, MMC (Metal Matrix Composites), high silicon aluminum and many other abrasive materials (Note: True diamond coatings should not be used while machining steels. More heat is generated while cutting steels and thus causes chemical reactions that break down the bonds that hold this coating to the tool).

Coatings for hard milling, tapping and drilling all vary and are application-specific. Also available are multi-layer coatings that chip to the next layer instead of the tooling substrate, providing a further increase in tool life.


Here you can compare the most common coating and the effect obtained

 Constituent                                        Property Effect

Co, Ni , Fe binder materials     Toughness, corrosion resistance
WC hard particles                Hardness, good wettability by cobalt
TiC, TiCN hard particles         Higher hardness, lower thermal conductivity and
                               lower solubility in iron alloys than WC
(Ta, Nb)C                        Improved high-temperature properties and thermal
                               shock resistance
VC, Cr3C2                        Grain growth inhibitors in WC-Co materials
Mo2C                             Improved wettability of TiCN by binder in cermets