Knowledge of tool coating technology
1、 Tool coating
A kind of thin film is formed on the tool surface by chemical or physical methods, so that the cutting tool can obtain excellent comprehensive cutting performance, thus meeting the requirements of high-speed machining; Since the advent of hard-coated tools in the early 1970s, chemical vapor deposition (CVD) technology and physical vapor deposition (PVD) technology have been developed one after another, opening a new chapter in history for the improvement of tool performance. Compared with uncoated tools, coated tools have obvious advantages: it can greatly improve the life of cutting tools; Effectively improve the cutting efficiency; Improve the machining accuracy and obviously improve the surface quality of the workpiece to be processed; Effectively reduce the consumption of tool materials and reduce the processing cost; Reduce the use of coolant, reduce costs, and benefit environmental protection.
2、 Characteristics of tool coating
1. The coating technology can greatly improve the surface hardness of the tool without reducing the tool strength. At present, the hardness achieved is close to 100GPa;
2. With the rapid development of coating technology, the chemical stability and high-temperature oxidation resistance of the film become more prominent, which makes high-speed machining possible.
3. The lubricating film has good solid lubrication performance, which can effectively improve the processing quality, and is also suitable for dry cutting;
4. As the final process of tool manufacturing, coating technology has little impact on tool accuracy and can be used for repeated coating process.
3、 Common coating
1. Titanium nitride coating: titanium nitride (TiN) is a general-purpose PVD coating, which can improve the hardness of the tool and has a higher oxidation temperature. The coating can be used for high-speed steel cutting tools or forming tools to obtain good processing results.
2. Chromium nitride coating: The excellent adhesion resistance of CrN coating makes it the preferred coating in processing that is prone to chip accumulation. After coating this almost invisible coating, the processing performance of high-speed steel tools or carbide tools and forming tools will be greatly improved.
3. Diamond coating CVD: diamond coating can provide the best performance for non-ferrous metal material processing tools, and is an ideal coating for processing graphite, metal matrix composites (MMC), high-silicon aluminum alloy and many other high-wear materials (note: pure diamond coating tools cannot be used for processing steel parts, because a large amount of cutting heat will be generated when processing steel parts, and chemical reaction will occur, which will damage the adhesive layer between the coating and the tool). [Metalworking WeChat, good content, worthy of attention]
4. Titanium nitride carbide coating: The carbon element added in the titanium nitride carbide (TiCN) coating can improve the hardness of the tool and obtain better surface lubricity. It is an ideal coating for high-speed steel tools.
5. Aluminum nitride titanium or titanium nitride aluminum coating (TiAlN/AlTiN): The aluminum oxide layer formed in the TiAlN/AlTiN coating can effectively improve the high-temperature machining life of the tool. This coating can be selected for cemented carbide tools mainly used for dry or semi-dry cutting. According to the different proportion of aluminum and titanium contained in the coating, AlTiN coating can provide higher surface hardness than TiAlN coating, so it is another feasible coating choice in the field of high-speed machining.
4、 Coating technology and tool coating knowledge
1. Titanium nitride carbide (TiCN): The coating has higher hardness than titanium nitride (TiN) coating. Due to the increase of carbon content, the hardness of TiCN coating is increased by 33%, and its hardness range is about Hv3000-4000 (depending on the manufacturer).
2. CVD diamond coating: CVD diamond coating with surface hardness up to Hv9000 has been applied to tools more mature. Compared with PVD coated tools, the life of CVD diamond coated tools has increased by 10-20 times. The high hardness of diamond coated tools can increase the cutting speed by 2-3 times than that of uncoated tools. The CVD diamond oxidation temperature is the temperature at which the coating begins to decompose. The higher the oxidation temperature is, the more favorable it is for cutting at high temperature. Although the hardness of TiAlN coating at room temperature may be lower than that of TiCN coating, it has been proved that it is much more effective than TiCN coating in high-temperature processing. The reason why TiAlN coating can still maintain its hardness at high temperature is that it can form a layer of aluminum oxide with CNC micro-signal cncdar between the tool and the chip. The aluminum oxide layer can transfer heat from the tool to the workpiece or chip. Compared with high-speed steel tools, the cutting speed of cemented carbide tools is usually higher, which makes TiAlN the preferred coating for cemented carbide tools. The PVDTiAlN coated stone coated tools are usually used for cemented carbide drill bits and end milling cutters, and become a good choice for cutting and processing of non-ferrous and non-metallic materials. Metal processing WeChat, with good content, deserves attention.
3. The hard film on the tool surface has the following requirements for materials: ① high hardness and good wear resistance; ② Stable chemical performance, no chemical reaction with workpiece material; ⑧ Heat resistance and oxidation resistance, low friction coefficient, firm adhesion with the substrate, etc. It is difficult for a single coating material to fully meet the above technical requirements. The development of coating materials has gone from the initial single TiN coating and TiC coating to the development stage of TiC-A12O3-TiN composite coating and TiCN, TiAlN and other multi-component composite coatings. Now the latest development of TiN/NbN, TiN/CN and other multi-component composite film materials has greatly improved the performance of tool coating.
4. In the manufacturing process of coated tools, the selection is generally based on the hardness, wear resistance, high temperature oxidation resistance, lubrication and adhesion resistance of the coating, among which the oxidation resistance of the coating is the technical condition most directly related to the cutting temperature. The oxidation temperature refers to the temperature at which the coating begins to decompose. The higher the oxidation temperature is, the more favorable it is for cutting under high temperature conditions. Although the hardness of TiAlN coating at room temperature may be lower than that of TiCN coating, it has been proved that it is much more effective than TiCN coating in high-temperature processing. The reason why TiAlN coating can still maintain its hardness at high temperature is that it can form a layer of alumina between the tool and the chip. The alumina layer can transfer heat from the tool to the workpiece or chip. Compared with high-speed steel tools, the cutting speed of cemented carbide tools is usually higher, which makes TiAlN the preferred coating for cemented carbide tools. This PVDTiAlN coating is usually used for cemented carbide drills and end mills
5. From the perspective of application technology: in addition to cutting temperature, cutting depth, cutting speed and coolant may have an impact on the application effect of tool coating.
5、 Progress of common coating materials and superhard coating technology
Among hard coating materials, TiN is the most mature and widely used. At present, the use rate of TiN-coated high-speed steel tools in industrial developed countries has accounted for 50-70% of high-speed steel tools, and the use rate of some complex tools that cannot be reground has exceeded 90%. Due to the high technical requirements of modern metal cutting tools, TiN coating is increasingly unable to adapt. The oxidation resistance of TiN coating is poor. When the service temperature reaches 500 ℃, the film is obviously oxidized and ablated, and its hardness cannot meet the requirements. TiC has high microhardness, so the material has good wear resistance. At the same time, it is firmly attached to the substrate. When preparing multi-layer wear-resistant coating, TiC is often used as the bottom film in contact with the substrate. It is a very common coating material in coated tools.
The development of TiCN and TiAlN has further improved the performance of coated tools. TiCN can reduce the internal stress of the coating, improve the toughness of the coating, increase the thickness of the coating, prevent the diffusion of cracks, and reduce tool breakage. Setting TiCN as the main wear layer of coated tools can significantly improve the tool life. TiAlN has good chemical stability, oxidation resistance and wear resistance. When machining high alloy steel, stainless steel, titanium alloy and nickel alloy, the service life of TiAlN coated tools is 3-4 times longer than that of TiN coated tools. If there is a high concentration of Al in the TiAlN coating, a thin layer of non-product A12O3 will be formed on the coating surface during cutting, forming a hard inert protective film. The coated tool can be more effectively used for high-speed cutting. Oxygen-doped titanium nitride carbide TiCNO has high microhardness and chemical stability, which can produce the effect equivalent to TiC+A12O3 composite coating. Metal processing WeChat, with good content, deserves attention.
Among the hard film materials mentioned above, there are three kinds with microhardness HV exceeding 50GPa: diamond film, cubic boron nitride CBN and carbon nitride.
Many diamond films are deposited at a temperature between 600 ℃ and 900 ℃, so this technology is often used to deposit diamond films on the surface of cemented carbide tools. The commercialization of diamond carbide tools is a major achievement of coating technology in recent years.
CBN is second only to diamond in hardness and thermal conductivity, and has excellent thermal stability. It will not oxidize when heated to 1000 ℃ in the atmosphere. CBN has extremely stable chemical properties for ferrous metals. Unlike diamond, which is not suitable for processing steel, it can be widely used for finishing and grinding of steel products. In addition to its excellent wear resistance, CBN coating can also process heat-resistant steel, titanium alloy and quenched steel at a relatively high cutting speed, and can also cut hardened roll with high hardness, carbon-doped quenched material and Si-Al alloy with very serious tool wear. CVD and PVD are the main methods to synthesize CBN films in low pressure gas phase. CVD includes chemical transport PCVD, hot filament assisted heating PCVD, ECR-CVD, etc; PVD includes reactive ion beam plating, reactive evaporation plating, laser evaporation ion beam assisted deposition, etc. There is still a lot of work to be done in basic research and application technology of CBN synthesis technology, including reaction mechanism and film-forming process, plasma diagnosis and mass spectrometry analysis, determination of optimal process conditions, development of high-efficiency equipment, etc.
Carbon nitride may have the hardness of diamond or more. The success of synthesizing carbon nitride is a very outstanding example of molecular engineering. As a superhard material, carbon nitride is expected to have many other valuable physical and chemical properties. The study of carbon chloride has become a hot topic in the field of material science in the world.
The fourth period of the periodic table of elements is the VB group element, which is a rare metal with high melting point. Element symbol V, atomic number 23, relative atomic mass 50.9415, solid at room temperature, gray in powder form, and steel gray in dense form.
In 1801, the Mexican mineralogist A.M. DelRio discovered a new element while studying limonite. Its chemical properties are similar to chromium and uranium. Its salt turns red when heated in acid, so it was named "erythronium", which means "red element", but was later mistaken for alkaline lead chromate. In 1831, the Swedish chemist N.G. Sefstrom discovered a new element when smelting pig iron. Its compounds were beautiful and colorful, so it was named Vanadium after the Swedish goddess "Vanadius". Vanadium was used as a colorant in 1860. In 1867, the British chemist H.E. Roscoe reduced VCl with hydrogen: for the first time, pure silver gray metal vanadium powder was obtained. In 1869, France studied the use of vanadium as an alloying agent in the production of armor plates. In 1896, vanadium was used as a special steel additive in Europe. Vanadium was used as a catalyst in 1870. Vanadium-containing alloy steel was used as raw material for automobile industry around 1905. In 1927, J.W.: Marden and M.N. Rich of the United States obtained industrial vanadium by calcium-thermal reduction in electric furnace.
China's vanadium industry started in the 1950s. In 1958, the vanadium extraction workshop of Jinzhou Ferroalloy Plant was restored and expanded, and the Vanadium-bearing iron concentrate in Damiao, Chengde, was used as the raw material for vanadium extraction. Since 1960, other vanadium extraction plants in China have been built and put into operation. In the 1970s, Panzhihua Iron and Steel Company was built and put into operation. Since then, China's vanadium industry has entered a new historical period. By the 1980s, China had become one of the major vanadium producers in the world and could produce various vanadium products. The promotion and application of vanadium has also developed rapidly.
Properties Vanadium has good plasticity and malleability. It can be made into sheet, drawn into wire and processed into foil at normal temperature. However, a small amount of impurities, especially interstitial elements (such as carbon, hydrogen, oxygen and nitrogen), will significantly affect the physical properties of vanadium. If vanadium contains 0.01% hydrogen, it will cause embrittlement and reduce plasticity; The melting point increases to 2458K when the carbon content is 2.7%. Vanadium has high melting point, high hardness, high resistivity, weak paramagnetism, low linear expansion coefficient, and its elastic modulus and density are similar to those of steel, which can be used as structural materials. The main physical properties of vanadium are shown in Table 1.
Vanadium is a transition element. The outer electron layer configuration of its atom is [Ar] 3d34s2, and all five electrons can participate in bonding. Therefore, vanadium is a variable valence element, with 8 valence states from - 3 and - 1 to+5, of which the vanadium compound with+5 valence is the most stable. The compounds with positive pentavalent vanadium have oxidizing properties, and the compounds with low valence vanadium have reducing properties. The lower the valence, the stronger the reducing properties of vanadium compounds, so V3+and V2+are strong reducing agents. Vanadium is easy to form complexes with many ligands.
Vanadium has 9 isotopes with mass numbers ranging from 46 to 54. 51V is a stable natural isotope. The corrosion resistance of vanadium is second only to niobium and tantalum. At room temperature, vanadium does not react with air, water and alkali, but can form compounds with most nonmetallic elements such as carbon, silicon, nitrogen, oxygen, sulfur, chlorine, bromine, etc. at high temperature. Vanadium is resistant to seawater corrosion, as well as hydrochloric acid, dilute sulfuric acid and alkali solution. Vanadium does not react with other halogenated acid at room temperature except for slowly reacting with hydrofluoric acid. Vanadium reacts with oxyacids (hot concentrated sulfuric acid, hypochloric acid, nitric acid and aqua regia) to form vanadate.
The common vanadium compounds include oxides, halides, ammonium salts, sodium salts and oxysalts. The properties of these compounds are often affected by the change of vanadium valence state.
The oxides of vanadium oxide mainly include V205, V02, V203 and V0. Their properties are listed in Table 2.
The chemical stability of vanadium halides decreases with the increase of vanadium valence; The chemical stability of vanadium halides with the same valence is gradually weakened from fluoride to iodide. The main properties of V3+and V5+halides are listed in Table 3 and Table 4.
Ammonium salts mainly include ammonium metavanadate (NH4VO3), ammonium vanadate [(NH4) 3VO4 or 2 (NH4) 20 • 3V205 • nH20], and ammonium polyvanadate, among which ammonium metavanadate is the most important. Ammonium metavanadate is white or light yellow crystal, which begins to decompose when heated to 473K, and its density is 2304kg/m3. Ammonium metavanadate decomposes into NH3 and V205 in oxygen-enriched air at 523K; It is decomposed into (NH4) 2O • 3V205 and V204 • 5V205 in air at 523~613K, and into NH3 and V205 at 693~713K; V205 was obtained by oxidation in pure oxygen atmosphere at 583-598K temperature. Ammonium metavanadate is heated to 473K in hydrogen to generate (NH4) 2O • 3V205, (NH4) 2O • V204 • 5V205 at 593K, V2013 and V203 at 673K, and V204 at 1273K. Ammonium metavanadate forms (NH4) 2O • V204 • 5V205 at 623K in CO2, N2 and Ar atmosphere, and V6013 at 673-773K. Ammonium metavanadate generates (NH4) 2O • 3V205 when water vapor is passed and the temperature reaches 498K.
The compounds in sodium vanadium-sodium salt such as V205-Na20 binary system phase diagram include NaV6015, Na8V24063, NaV03, Na4V207 and Na3V04. Sodium metavanadate (NaV03), sodium pyrovanadate (Na4V2O7), sodium orthovanadate (Na3V04) and sodium polyvanadate are common salts in the phase diagram of NaV03-V205 system.
Oxygenate Vanadium Oxygenate, except for alkali metal and alkaline earth metal vanadate, which are easily soluble in water, other salts have low solubility. Vanadium oxide salts commonly include sodium salt, ammonium salt, calcium salt, iron salt, etc. According to the aggregation state of vanadium, it can be divided into orthovanadate (V043 -), pyrovanadate (V2074 -), metavanadate (V03 -, V40124 -) and polyvanadate (V40124 -, V60162 -, V100182 -, V120312 -), etc.
Vanadium alloys and additives The commonly used vanadium alloys include ferrovanadium, vanadium-aluminum, molybdenum vanadium-aluminum alloy, silicon manganese vanadium-iron alloy, vanadium carbide, vanadium nitride and other special alloy additives. The standards of ferrovanadium alloy in different countries are different. Generally, there are six grades including 40%~80% vanadium. There are four vanadium aluminum alloys containing 55%, 65%, 75% and 85% vanadium. Molybdenum-vanadium-aluminum alloy generally contains 23% - 25% vanadium, 24% - 26% molybdenum and 48% - 53% aluminum. Silicon manganese ferrovanadium generally contains vanadium>42%, silicon<7% and manganese<4.5%. Vanadium carbide generally contains 82% - 86% vanadium and 10.5% - 14.5% carbon. Vanadium nitride contains 78% - 82% vanadium, 10% - 12% carbon and more than 6% nitrogen.
Toxic vanadium and its compounds have certain toxicity, which generally increases with the increase of vanadium valence. Skin is prone to allergic reaction when exposed to vanadium. Industrial contact is easy to cause respiratory tract diseases, including paroxysmal dry cough with hemoptysis and irritation symptoms of eyes, nose and throat. Hemoglobin and red blood cells increase temporarily and then decrease, and anemia occurs, sometimes conjunctivitis, severe throat irritation, stubborn dry cough, diffuse rale and bronchospasm, sometimes skin itching and other symptoms. When soluble vanadium enters the circulating blood, the lethal dose of 70kg body weight healthy people is only 30mg V205 (that is, the body weight of a woman is O.43mg), and the threshold limit value of some vanadium compounds is: V205 dust O.5mg/m3, smoke dust or ultrafine powder O.1mg/m3. The effects of vanadium on animal neurophysiology (including central nervous system dysfunction and cardiovascular disease) cannot be ignored. Vanadium has no obvious effect on the metabolism of animals. A small amount of vanadium salt in the mouth and eyes has little toxicity. It can also reduce the cholesterol of young people and animals; Intravenous injection is very toxic. Acute exposure to and absorption of high concentration of vanadium will cause vasoconstriction, leading to congestion and hemorrhage in some parts of the body and damage the liver, kidney, heart and brain. Vanadium has been used in medicine to treat anemia, chlorosis, tuberculosis and diabetes. The maximum allowable vanadium content in domestic drinking water is below 0.1mg/L.
The application of vanadium has excellent alloy properties and catalytic properties, and is widely used in iron and steel industry and chemical industry. 90% of the world's vanadium is used in the steel industry, 5% in the chemical industry, and the rest in non-ferrous alloys and other fields. Vanadium is added into steel to form vanadium carbide and vanadium nitride particles, which can strengthen the steel dispersion, inhibit the growth of austenite grain and improve the steel properties. The strength of steel can be increased by 10%~20% when vanadium is added by 0.05%~0.1%, and the casting and welding properties of steel can be improved, and the weight of machine parts can be reduced. High speed tool steel containing 1%~5% vanadium has good plasticity and toughness in addition to high wear resistance. Vanadium is mainly used in the chemical industry to manufacture various catalysts and ceramic pigments. The application of vanadium in nonferrous alloys is mainly used to produce vanadium aluminum alloys. Vanadium-aluminum alloys have high yield strength and impact strength, and are used as materials for aircraft and aerospace vehicles. The consumption of vanadium in this area is increasing year by year, and the United States has now risen to 7% - 10% of the total consumption of vanadium. In addition, single crystal vanadium can be used as a material in the electronic industry.
Resource vanadium deposits can be divided into magmatic secretion deposits, vanadium-bearing hydrothermal deposits, weathering accumulation residual deposits and sedimentary deposits according to their ore-forming reasons. The crustal abundance of vanadium is 136 × 10-4%, seawater contains vanadium 1 × 10-7％。 In vanadium ore, vanadium is mainly associated with iron, titanium, uranium, molybdenum, copper, lead, zinc, aluminum, or carbonaceous rock and phosphate rock. Recently, petroleum minerals with high vanadium content have also been found. Common vanadium minerals are listed in Table 5.
Vanadium-vanadium-titanomagnetite is the main resource of vanadium production, which can comprehensively utilize vanadium, iron and titanium. The main vanadium deposits in the world are the vanadium-titanium magnetite in Bushveld, central Transvaal, South Africa, with a reserve of about 2 billion tons; Ural and Kachkanar of the former Soviet Union; Otanmeki and Mustavara, Finland; Wyoming Iron Mountain, USA; Vanadium-titanium magnetite from Panzhihua, Sichuan, China and Chengde, Hebei. The composition of some important vanadium-titanium magnetite raw ore and vanadium-bearing iron concentrate in the world is listed in Table 6.
Other vanadium resources include uranium vanadate, vanadate, phosphate rock, asphalt, petroleum, graphite ore and vanadium-bearing mica, as well as stone and coal mines in Sichuan, Hubei, Hunan, Jiangxi, Zhejiang, Guizhou, Guangxi and other provinces in the middle and lower reaches of the Yangtze River in China. At present, the proven vanadium resources in the world are 4.36 million tons and the prospective reserves are 16.62 million tons. The resource reserves of major vanadium producing countries are shown in Table 7.
The extraction metallurgy of vanadium metal from vanadium containing raw materials mainly includes vanadium extraction and vanadium metal preparation. Sometimes in order to produce high-purity vanadium metal, it is necessary to add the vanadium refining process. Vanadium regeneration also belongs to vanadium extraction metallurgy. The principle flow of vanadium extraction metallurgy is shown in the figure. In the process of vanadium extraction metallurgy, vanadium extraction is the most important. Vanadium extraction is the process of preparing vanadium compounds from vanadium containing raw materials. Due to the small amount of vanadium concentrate and the main raw material of vanadium is vanadium-titanium magnetite, the process of extracting vanadium from vanadium-bearing molten iron produced by smelting vanadium-titanium magnetite plays an important role in vanadium extraction metallurgy. This process is that vanadium-titanium magnetite first enters the ironmaking furnace, vanadium is reduced into molten iron, and vanadium-containing molten iron is blown to produce vanadium slag; Then vanadium is extracted from vanadium slag. Vanadium slag and various vanadium-containing materials are generally treated by roasting, leaching and solution purification to obtain a relatively pure vanadium-containing solution. Then V205 and V203 are prepared from the relatively pure vanadium-containing solution Ammonium metavanadate, calcium vanadate, iron vanadate and other vanadium compounds. The vanadium compounds obtained from vanadium extraction are mostly sold as products except for some V205 as raw materials for the production of vanadium metal. V205 is reduced by calcium, aluminum or carbon to produce crude vanadium, which can be refined to obtain pure vanadium. V205 is widely used as raw material for producing ferrovanadium, which is an important alloy additive for alloy steel production.