Capítulo 4: Construcción de la solución propuesta
4.7. Conclusiones
Much research has previously been carried out to investigate ultra-hard tool wear characteristics during machining with different workpiece materials, demonstrating that the severity and mechanism of wear in ultra-hard tools vary with the type of workpiece material selected [93]-[94]. The typical ultra-hard material wear modes have been categorized into four groups: normal wear, chipping, chip dragging and fracture of the cutting edge [97]. It has been demonstrated that with an increase in grain size an improved abrasion resistance can be achieved but the edge quality is higher for finer grain size PCD [40], [98], thus depending on the harsh/moderately harsh test conditions, the fine PCD could be preferred for grinding and polishing applications where better surface quality is required. An example of the effect of the grain size dimension is reported in Figure 2. 22 where the typical abrasive wear rate and tool edge roughness are shown for a fine, medium and coarse grained PCD.
Figure 2. 22: Influence of the grain size on the edge roughness and on the wear rate [98].
36 Much research has been done on the evaluation of the wear properties of PCD composites, and it was indicated that because of the extreme wear resistance of the composites, standard wear testing procedure were not suitable [99], [100]. A good approach for cutting test of super-abrasive materials has been proposed in 2005; in the proposed set up a vitrified bonded alumina grinding wheel was used as workpiece [101]; during the cutting test (Figure 2. 23), two were the main forces considered between tool and workpiece: the cutting force and the normal force required to maintain the specific depth of cut; the effect of the latter is to accompanying fracturing and cracking wear mechanisms [102] [105].
Figure 2. 23: Set up of the wear test of superabrasive material on a vitrified bonded alumina grinding wheel workpiece [101].
A cutting length up to 4000 m and a depth of cut of 0.3 mm were adopted to test the wear of WC and two different grain size PCD (respectively 13.6 and 15.1 micrometers grain size). In this study the wear was empirically evaluated measuring the weight loss of the workpiece and of the PCD inserts during the test. As well known, PCD is not thermally stable having a complex mix of phases and binder/hard phase bulk volume percentage variable depending on the specimen microstructural factors, for this reason the results of the wear test on PCD have indicated different wear behaviours [102]. This study indicated that two cuts of 75 m were enough to cause abrasive wear onto the insert. In particular, it has been proved that the PCD with finer grain size (13.6 micrometers) worn out quicker than the medium size one (15.1 micrometers). This behaviour has been
37 explained considering the major factors affecting cutting performance; in particular reaction forces on the tool, stress distribution in the tool and the workpiece, cutting speed and depth of cut, temperature at the tool (if test is performed in dry condition) and removal of swarf in the area in front of the diamond grain [102]. In relation to this, a model on the chip formation in cutting stone with diamond was previously proposed in 2003, and the schematic of the chip formation process is depicted in Figure 2. 24.
Figure 2. 24: Schematic model of the chip formation process during the interaction of a single grain diamond and a stone workpiece [105].
The model for chip formation allowed identifying the main aspects of the wear of diamond tools, indicating that micro-mechanisms are responsible for wear processes: while at the front of an individual diamond grain severe abrasive wear was found [102], [103], [105], less severe abrasive mechanisms occur on the flank/rake surfaces of the cutting element because the force is distributed along the flank surface [102]. Two are the main consequences of wear mechanisms in PCD inserts: either delamination of the PCD composite from its WC substrate (due to weak bonding), or premature abrasive wear with cracking due to defects in the synthesis process [102].
Regarding the reported literature on wear of PCBN tools, some researches have been conducted on the PCBN tool wear mechanisms and this lead to identify five wear mechanisms: abrasion, fatigue, adhesion, dissolution/diffusion and tribochemical processes [106], [107].
Polycrystalline cubic boron nitride (PCBN) has been used as cutting tool in hard turning of steel, producing smooth and uniform finished surfaces,
38 although deterioration of the tool surface is a commonly encountered problem due to the chip type formation [54]. An example of the performance of a low-CBN PCBN tool is depicted in Figure 2. 25. It has been demonstrated that low-CBN grades found application in continuous and semi-interrupted hard turning because of their reduced flank wear;
high-CBN grades tend to resist more to the interrupted machining while wearing rapidly at higher cutting speeds [56]. The type of wear depends upon the continuity or interruption of the test and on the CBN percentage in the PCBN tool: abrasive wear has been found in continuous cutting conditions, while chipping was found as an addition to abrasion in the interrupted cutting [34], [40], [54].
Figure 2. 25: Performances of low-CBN PCBN grades in interrupted turning machining of steel [40].
When considering the wear on a PCBN cutting insert, the combination of high temperature and stresses during testing gives a certain severity level of wear depending also on the geometry of the cutting tools, mechanical and thermal loading during testing. As a general guidance, researchers have identified flank and crater wear, chipping, nose wear and thermal shock cracks as frequent wear patterns in cutting insert made of PCBN [108].
While there is still a lack in fully understanding the wear progression mechanisms of composite ultra-hard materials, several researches have reported on the wear performances of electroplated diamond abrasive tools. Most of them have demonstrated a progressive increase in cutting
39 forces and a dulling of the abrasives mainly caused by attritious wear [38], [66], [67].
In spite of all the reported literature in the performances of both low and high-CBN grades in conventional machining, no study has been done in wear/cutting tests of these grades manufactured at a micro-scale and arranged in arrays, simulating grinding-type operations.