What is waterjet machining?
Waterjet machining can be referred to as a material removal process which uses a high velocity stream of water or a mixture of water and abrasives.
Waterjet machining can be categorized into:
- Pure water jet: Soft materials such as paper products, sealing materials, plastics, foams, textiles, and food can be cut via pressurized stream of water.
- Abrasive water jet: Very hard materials such as glass and metal are difficult for a waterjet to penetrate on its own. Abrasive water jet uses a mixture of water and abrasive to more effectively cut through materials. In the earlier days abrasive particles such as fine grade of sand, crushed walnut hulls were used, today the most commonly used abrasives are garnet, aluminum oxide and silicon carbide. Soluble abrasives can also be introduced into the jet stream which after striking the target get dissolved in the water avoiding separate and costly cleanup practices. 
A high-pressure water pump is used to pressurize the water. This pressurized water then travels through a high pressurized plumbing into the nozzle. At the nozzle, abrasives can be introduced which later on gets mixed into the mixing tube and ejects out of the nozzle at high speed. This high speed abrasive/pure water jet can be used in rock cutting, cutting in manufacturing industries, surface treatment (to remove coatings, paint, deposits, or even rust) and in food industry (cutting meats and fish, frozen foods, vegetables, and even prepared foods such as cakes, pastries, and bread). 
Tribological components of abrasive waterjet
- Mixing chamber and focusing/mixing tube: A critical component of AWJ systems is the mixing tube where the high-velocity jet of water is mixed with the cutting abrasive. The mixing chamber is susceptible to normal wear and has about a 500-hour lifespan. Wearing of the mixing tube is a serious problem in abrasive water jet machining. Attention should be paid on selection of mixing tube material as it has a direct effect on the performance of AWJ.
- Nozzle: Increased wear of the AWJ nozzle makes the clearance between the abrasive mixture and the nozzle larger which causes incomplete mixing of the abrasive particles with the high velocity waterjet which results in deterioration of AWJ performance and workpiece geometry. A worn nozzle should be replaced with the new one at the right time.
- Pneumatic valves: The pneumatic on/off valve opens and closes to control water flow to the cutting head. When the valve is worn, water will drip from the cutting head. Polymer composite materials with hard surface coating based on complex nickel alloy have been tested for manufacturing of advanced hydraulic and pneumatic valves.
- Pipelines and fittings: The high-pressure lines and fittings experience normal wear and have to be replaced at regular intervals. Safety of the operators could be at risk considering the pressure being pumped through it. Ceramic pipe and fittings prove to be extremely wear resistant and highly corrosion-resistant.
- Pump: High pressure pumps are subjected to various types of wear processes such as erosion and corrosion -Erosion is the wear and tear of the pump internal parts by suspended solid particles contained in the fluid being pumped. The most affected parts are: wear rings, shaft sleeves, packing, mechanical seal faces, lip seals, the pump casting, and the impeller. Elimination of presence of hard water and proper water conditioning could solve these issues. Corrosion is caused by a chemical or electrochemical attack on the surface of the metals. It is increased when there is an increase in temperature and/or presence of oxygen in the fluid or the surface of the fluid. Cavitation, erosion, and high fluid velocity advance the corrosion process. 
Wear prevention of abrasive waterjet nozzles
Umang Anand and Joseph Katz through their research work introduced a novel solution for preventing nozzle wear in Abrasive Water Suspension Jets used for jet cutting. A porous material was used to make nozzle and was surrounded by a reservoir containing a high viscosity lubricant. Same pressure was used for the lubricant reservoir which drove the flow in the nozzle. The pressure difference created due to the high-speed flow in the nozzle, continuously forced the lubricant through the porous medium, resulting in the formation of a thin film of high viscosity fluid on the interior walls of the nozzle. This lubricant film as a result protected the walls of the nozzle from the abrasive wear. 
Kovacevic took a different approach and developed a wear sensor system for direct and almost on-line tracking the wear of an abrasive waterjet (AWJ) nozzle the sensing system can detect the direction of the wear propagation. The sensing system can detect the direction of the wear propagation also it will provide the necessary information about the positioning control in order to compensate the increasing in inner diameter of the nozzle. 
K.A.Schwetz and team investigated wear of boron carbide ceramics used a nozzle material for waterjet. The material performed excellently when very hard abrasives such as alumina are used. The combination B4C nozzle-Al2O3 abrasive proved economic for difficult cutting jobs. 
Wear prevention of abrasive waterjet mixing/focusing tube
Water and abrasive mix exit the focusing tube in the form of a coherent jet providing the machining. Wear of focusing tube can be of following types: Weight loss, incidence of wear patterns along the inner surface, change of the outlet geometry and increase in exit diameter. J. BARALIĆ and B. NEDIĆ in their research work suggested that longer duration of cutting induces the increase in the diameter of the focusing tube outlet, this correlation being almost linear. 
In another research work conducted by M Hashish, accelerated wear tests were conducted on a wide range of candidate tool materials for mixing tube. Results showed that tungsten carbide grades exhibited greater longevity due to binder in tungsten carbide which acts as a critical factor to controls the lifetime of tungsten carbide mixing tubes. Further investigations revealed that the wear mechanisms along the mixing tube change from erosion by particle impact at the upstream sections to abrasion at the downstream sections.
- Water Jet Cutter: A Tribological Tool for Various Uses in Industry, MANE 6963: Friction and Wear of Materials, Term Research Project, Joseph Giedra
- Tribology in Water Jet Processes by Seiji Shimizu DOI: 10.5772/15804
- Prevention of Nozzle Wear in Abrasive Water Suspension Jets (AWSJ) Using Porous Lubricated Nozzles by Umang Anand and Joseph Katz DOI; 10.1115/1.1491977
- A new sensing system to monitor abrasive waterjet nozzle wear by R.Kovacevic DOI: 10.1016/0924-0136(91)90211-V
- Wear of boron carbide ceramics by abrasive waterjets by K.A.Schwetz, L.S.Sigl, J.Greim and H.Knoch DOI: 10.1016/0043-1648(95)90019-5
- A review on nozzle wear in abrasive water jet machining application by Syazwani, H. & A. G, Mebrahitom & Azhari, Azmir. DOI:10.1088/1757-899X/114/1/012020
- Focusing Tube Wear and Quality of the Machined Surface of the Abrasive Water Jet Machining by J. BARALIĆ, B. NEDIĆ, V. MARUŠIĆ
- Observations of Wear of Abrasive-Waterjet Nozzle Materials by M. Hashish DOI: 10.1115/1.2928861
- Featured image credits: Description: English: Retro Systems waterjet CNC cutting machine. Date: 16 April 2013, 03:02:13, Source: Retro Systems LLC, www.retrosystems.com, Author: Steve Brown Photography
- Szyniszewski, S., Vogel, R., Bittner, F. et al. Non-cuttable material created through local resonance and strain rate effects. Sci Rep 10, 11539 (2020). https://doi.org/10.1038/s41598-020-65976-0
Harshvardhan Singh works as a Senior Service Engineer at a mining firm in India. He is currently working into oil analysis field. Has worked in the filed of tribology and lubrication and loves to write about the same.