A knife, at its simplest, is a single sharp blade; the rest is all detail. The knife is one of the oldest and still most used survival tools in existence; its history stretches well beyond recorded time. It is both tool and weapon.
Drop point is a term used to describe a knife blade that slopes on the spine of the blade from the handle of the knife to the tip of the blade. This allows the spine of the blade (where the blade is thicker, and thus stronger) to continue forward to the tip of the blade.
The curve on the top of a drop-point blade is always convex, which is what distinguishes it from a clip point blade. Drop point is a common design for hunting knives.
A serrated blade is a type of blade used on saws and on some knives or scissors. Also known as a dentated or toothed blade.
A serrated blade has a cutting edge that has many small points of contact with the material being cut. By having less contact area than a smooth blade, the applied force at each point of contact is relatively greater and the points of contact are at a sharper angle to the material being cut. This causes a cutting action that involves many small splits in the surface of the material being cut, which cumulatively serve to cut the material along the line of the blade.
Cuts made with a serrated blade are typically less smooth and precise than cuts made with a smooth blade. Serrated blades are also more difficult to sharpen using a whetstone or rotary sharpener than a smooth blade.
Knives are available in several materials each with their own unique properties to be considered.
Carbon steel, also called plain carbon steel, is steel where the main alloying constituent is carbon. The AISI defines carbon steel as: "Steel is considered to be carbon steel when no minimum content is specified or required for chromium, cobalt, columbium [niobium], molybdenum, nickel, titanium, tungsten, vanadium or zirconium, or any other element to be added to obtain a desired alloying effect; when the specified minimum for copper does not exceed 0.40 per cent; or when the maximum content specified for any of the following elements does not exceed the percentages noted: manganese 1.65, silicon 0.60, copper 0.60." Needs reference link
The term carbon steel may also be used in reference to steel which is not stainless steel; in this use carbon steel may include alloy steels.
Steel with a low carbon content has properties similar to iron. As the carbon content rises, the metal becomes harder and stronger but less ductile and more difficult to weld. In general, higher carbon content lowers the melting point and its temperature resistance. Carbon content influences the yield strength of steel because carbon molecules fit into the interstitial crystal lattice sites of the body-centered cubic (BCC) arrangement of the iron molecules. The interstitial carbon reduces the mobility of dislocations, which in turn has a hardening effect on the iron. To get dislocations to move, a high enough stress level must be applied in order for the dislocations to "break away". This is because the interstitial carbon atoms cause some of the iron BCC lattice cells to distort. 85% of all steel used in the U.S. is carbon steel.
In metallurgy, stainless steel is defined as a steel alloy with a minimum of 11.5% chromium content by mass. Stainless steel does not stain, corrode or rust as easily as ordinary steel (it "stains less"), but it is not stain-proof. It is also called corrosion resistant steel when the alloy type and grade are not detailed, particularly in the aviation industry. There are different grades and surface finishes of stainless steel to suit the environment to which the material will be subjected in its lifetime. Common uses of stainless steel are cutlery and watch straps.
Stainless steel differs from carbon steel by amount of chromium present. Carbon steel rusts when exposed to air and moisture. This iron oxide film is active and accelerates corrosion by forming more iron oxide. Stainless steels have sufficient amount of chromium present so that a passive film of chromium oxide forms which prevents further corrosion.
Most commercially available, mass-manufactured knives are made of stainless steel.
A ceramic knife is a knife made out of very hard ceramic, often zirconium oxide (ZrO2). These knifes are generally produced by compacting Zirconia powder using high pressure presses which apply a pressure of around 300 tons to produce blade shaped blank. These blanks are very brittle and fragile which can be shattered by a slight blow and special binders are used to retain the shape of the blank till the firing process. Like all ceramics these are hardened by firing at around 1400 degree celsius for about 2 days in kilns. the result is a very tough and blunt blade which needs to be sharpened to get the desired cutting edge. The blades are sharpened by grinding the edges with a diamond dust coated grinding wheel.
Zirconia is very hard; it ranks 8.5 on the Mohs scale of mineral hardness, compared to 6 to 6.5 for hardened steel, and 10 for diamond, giving a very hard edge that rarely needs sharpening. However, when sharpening is needed, they cannot be resharpened the same way as steel blades, which are often sharpened with a ceramic whetstone. To sharpen the edge of a blade a material harder than the one that is being sharpened is required, and ceramic knives are usually sharpened with industrial grade diamond sharpeners.
Ceramic knives will not rust, leading to their use by SCUBA divers. They are also nonconductive and nonmagnetic, which can be useful for bomb disposal operations. Their chemical inertness to both acids and alkalis and their ability to retain a cutting edge upto ten times longer than forged metal knives, makes them a very best culinary tool for slicing and cutting through boneless meat, vegetables and fruits. Since they are very ridgid they cannot be used for chopping, cutting bones or frozen foods or prying open things which may cause the cutting edge to chip off or the blade to break free from the handle. The tips of these knives are resistant to rolling and pitting but may break when dropped to ground,
Several brands also offer a black blade made by an extra firing or sintering via hot isostatic pressing (HIP). This process turns the ZrO2 into zirconium carbide (ZrC). The transformation to zirconium carbide improves the toughness of the blade, the key limitation to using ceramics in knife blades.
Ceramic knives present a conceptual problem to the security industry since ceramics are not picked up by metal detectors. To solve this problem, many manufacturers of non-military knives include a quantity of metal in each knife to ensure they are detectable with standard equipment. Ceramic knives can be detected by extremely high frequency scanners (i.e. Millimeter wave scanner), although (as of 2006) these scanners are not yet in widespread use.
Five of the worldwide or US manufacturers that produce ceramic pocket or folding knives are Kyocera, Böker, Flint Knives, Shenzhen Hetiansheng Precision Ceramics, and Tachi Knives.