I will describe four methods which lead to different outcomes for sharpening this S110V knife. Many other approaches exist, but there are a finite number of final outcomes for microstructure and geometry of the cutting edge. Beyond geometry, the amount of damage to the steel below the apex will also be determined by sharpening technique. One mode of failure (or dulling) is the build-up of defects to the steel below the apex leading to micro-chipping. Minimizing the damage caused by sharpening may significantly improve initial edge retention. One of the attractions of this type of steel is that even after the initial edge fails, the blade usually transitions via micro-chipping to a long-lasting ceramic-knife geometry. Unlike traditional cutlery steels, the geometrically dull blade maintains slicing aggression for an extended period due to the micro-texture of the carbide surface. The goal here is to analyze the initial sharpened edge.
Ragged Burr Edge
The simplest and least effort is to sharpen at a single angle (in the vicinity of 30 degrees per side) with a single diamond plate or coarse silicon carbide stone. No effective deburring is performed, but instead increasingly lighter strokes at the sharpening angle are used to stand up the burr and apply a keen, burnished edge to the apex of that burr. This typically produces a 10-micron sized burr weakened by cracking of carbides that occurs in the flex-region near the apex. The very weakest regions will chip away with use but the slicing aggression of the ragged burr will remain. Despite the carbide damage, this is a sturdy burr and not one that can be removed by simply cutting into wood or stropping on anything other than diamond (or CBN) loaded material.
Another variation of this first method is to strop “ineffectively” to add keenness to the longest regions of the burr and break off some of the weakest regions. This will produce a good working edge with more than acceptable edge retention.
Exposed Carbide Edge
The second method is to sharpen as in the first method, but successfully remove the burr with some form of stropping or buffing, replicating the Spyderco factory edge. Alternatively, a very fine, muddy stone (eg 8k with slurry) can produce similar microstructure at the apex. This should allow the blade to be sharpened at a more aggressive (included) angle of 25-30 degrees and remove much of the sharpening-damaged steel from the apex in the follow-up step. This approach won’t produce push-cutting keenness, but should provide more than acceptable slicing aggression in many materials. The challenge with this approach is that the final result can vary strongly with the amount of matrix removal. Exposing more carbides, as does the factory edge, leads to a broad apex. A finer result, as shown below, can potentially be achieved by minimizing burr formation followed by careful carbide exposure.
Keen and Burr-Free Edge
The third method is to again sharpen as in the first method but remove the burr and damaged sub-apex material by sharpening at higher than the sharpening angle to create a micro-bevel or micro-convexing with a smooth diamond loaded strop. The goal is to remove not only the burr, but the damaged steel below the burr (the burr root). This is also the geometry produced by honing on the 40 degree Sharpmaker ceramic rods, albeit much cleaner results are produced by stropping on charged leather.
The fourth method is to try to avoid forming a problematic (flexing) burr altogether by sharpening at a relatively high angle of 40+ degrees inclusive. At these high angles, the burr can be typically removed by simply moving to higher grits and alternating sides frequently at the sharpening angle. This approach can produce good results at the apex, although the resulting geometry is likely too thick to perform well in cutting more rigid materials where wedging plays a role. In the longer term, as the edge chips away, leaving a ceramic knife type of edge, the thicker geometry will also hold back performance. This choice is often motivated by the unsubstantiated belief that the steel “can’t support” lower angles.
Each of these sharpening methods will have their proponents, and it is not my intention to rate or rank them. The purpose of the above information is to show that there is more to sharpening techniques than “finishing grit” or the non-physical “toothy” vs “polished” moniker. With long-term use and dulling, the evidence suggests that all of these starting points can transition to a long lasting ceramic-knife type geometry.
Dulling of the Keen-Edged Blade
It is an open question as to whether it makes sense to apply a keen edge to these high-carbide steels. As small as the carbides are, they are still large compared to the scale of a keen edge. And while carbides at the very apex can be shaped with diamond-loaded strops, a keen edge is primarily formed in the matrix steel. As a starting point, the following experiment explores the mechanism by which a keen edge in this very high carbide steel deteriorates with use.
Starting with the keen-edged blade described above, 1/4 inch sisal rope was slice cut using a single point on the blade and the deterioration monitored with occasional SEM imaging. After 500 cuts, the blade was honed on a Spyderco white ceramic to restore the edge, and that edge was dulled by the same 1/4 inch sisal rope cutting method.
The series of images below show cross-sections through multiple locations of the blade (in the cutting region) after 100, 300 and 500 slice cuts. Anecdotally, the performance was more than satisfactory for about the first 200 cuts and became unacceptable by the end of 500 cuts. Note that I was performing this test with the blade removed from the scales which would otherwise provide additional leverage. Dulling does not occur uniformly across the edge, and several locations were examined. Note that the image magnification is lower where the duller edges are imaged – comparison of the scale bar in the bottom right of each image with the apex dimension is required to appreciate the scale of dulling that has occurred.
Despite the blade being blunted after 500 cuts, microchipping was still minimal although the build-up of carbide fatigue and cracking is evident. This is likely due to the minimal flexing of the 40 degree (inclusive) microbevel. Also, the task (careful rope cutting) presumably doesn’t involve enough lateral deflection to produce microchipping before the apex is blunted. Again, wear to the apex does not occur uniformly along the edge.
The “shaving” level of keenness (sub 0.1 micron edge) at the apex was lost early within the first 100 cuts as wear and deformation to the matrix steel occurred. This is not at all surprising considering our experience with how delicate the edge of a razor blade or new utility knife blade are. It is remarkable that a sub-micron level of keenness was maintained in the matrix steel at the apex at 300 cuts. It appears that the carbides provide structure to the near-apex steel, slowing down deformation-type blunting until the carbides are severely damaged, softening the apex to allow blunting. Keeping in mind that this is a preliminary result, it does not appear that the wear-resistance of the hard carbides contributes directly to the performance and edge retention of the keen edge. If this is generally the case in this class of steels, it suggests that there is likely an optimal carbide concentration rather than “more is better” for keen edge retention. More specifically, we want the carbides to be isolated rather than clustered. Carbide damage consistently occurs more readily in clusters where two or more carbides impact one another with flexing of the steel, suggesting that a steel with better dispersed carbides (not clustered) would perform better in this application.
At this point the knife was “refreshed” by honing on the flat surface of the white ceramic rods of a Spyderco Sharpmaker, 10 light passes per side. This new microbevel is observed to have a remarkably keen edge considering the coarseness of the ceramic.
The refreshed edge was then used to slice the same 1/4 inch sisal rope at one point. Cross-sectioning shows that the particularly keen edge produced by the ceramic hone was shorter-lived compared to the initial sharpening. There was noticeable deterioration in cutting performance by the end of the first 100 cuts and cutting become difficult by the end of the second 100 cuts. This edge is more difficult to characterize by single point cross-sections as it is less uniform.
For final comparison, the knife was sharpened with a DMT Fine plate with minimal burr, followed by “burr removal” and carbide exposure using a Shapton Glass 8k stone with a small amount of slurry. The result is a moderately aggressive 30 degree bevel with minimal rounding or micro-beveling. Somewhat surprisingly, this particular edge deteriorated very quickly during the rope cutting; performing well for about 20 cuts but unable to cut cleanly through the rope by around 80 cuts. Anecdotally, this is consistent with the mixed results I have experience when experimenting with this approach.
The rapid deterioration in the above example may have two causes. First the micro-bevelling step likely introduced flex damage. This damage was not evident at the one location I analyzed. There is always a concern with micro-bevelling in that increasing the sharpening angle results in very high lateral pressure at the apex due to the very small contact area that exists before the new bevel is formed. Second, the micro-textured apex is not ideal for initiating the separation of fibers in the rope, demanding higher force to be applied to the blade with the increased force accelerating damage to the edge.
The results shown above are preliminary; however, they do highlight the importance of appropriate sharpening technique. More work is needed to determine the most efficient and appropriate approach. In summary, in this particular case, we cannot predict edge retention from geometry and steel type alone.
Although the carbides do not play a direct role in the performance of a keen edge, this steel has shown that when created with minimal burr-root sharpening damage, that keen edge can have excellent edge retention. The challenge is to form such an edge while minimizing carbide damage that leads to premature failure. Ideally, we want the knife to blunt through other processes at around the same rate that damage build-up leads to micro-chipping and the transition to the long-lasting ceramic edge geometry. These results also highlight the importance of the matrix hardness – it appears that the softer and more deformable the matrix, the faster the carbides crack.