3. Experimental results and discussion
3.1. Tool flank wear
3.1.1. CBN tools
The SEM image of the cutting edge of CBN-1 at Lc¼156 m is
shown in Fig. 4. Here, the cutting length Lc is the actual length
calculated by tool–workpiece contact length lc and multiple
number of cutting pulses as shown in Fig. 2. In the case of the
CBN tool, the tool flank wear of 25 mm or less was obtained even
when the cutting speed was as high as 600 m/min. According to the
above result and the fact that the geometry of the cutting edge did
not change, the CBN tools have excellent performance of wear
resistance in hardmilling regardless of the degree of CBN content.
3.1.2. Coated carbide tools
The change in flank wear width VB with cutting length Lc for four
types of K10-based coated carbide tools is shown in Fig. 5. VB was
defined as the peripheral flank wear width at which the tool
temperature was measured (h¼1.5 mm) as shown in Fig. 2. As is
obvious from the figure, the minimum tool wear VB¼20 mm at
Lc¼112 m was obtained for TiAlN/AlCrN-coated carbide tool, and
VB increased in the order of TiAlN-, TiCN- and TiN-coated carbide
tool. The wear resistance depends considerably on the hardness and
oxidizing temperature of the coating film material. As shown in
Table 3, the TiAlN/AlCrN-coating has the highest hardness and
oxidizing temperature and the TiAlN coating has similar characteristics.
Additionally, it was reported that aluminum in the coating
film is oxidized selectively under high temperature condition and
forms a thin layer of amorphous aluminum oxide, which protects
the coating from further oxidation [22]. Fig. 6 shows the SEM images
of the cutting edge of the TiN-coated carbide tool and the TiAlN/
AlCrN-coated tool at Lc¼112 m, where the VB of these tools were
the largest and the smallest, respectively. It is clear that the cutting
edge of TiN was irregularly formed by flaking of the coating film and
wear of base material. According to the above consideration, TiAlNand
TiAlN/AlCrN-coated carbide tools offer superior performances
in relatively low-speed hardmilling in view of wear resistance.