CACT has a wide range of equipment and facilities to cover virtually all areas of Thermal Spray Coatings, right from the Coating formation to their testing and characterization. The Centre also offers a unique combination of Computing facilities to carry out the Numerical Modelling work.
The centre has a wide range of thermal spraying equipment as listed below. The photographs of some of these are also shown subsequently along with a suitable explanation:
The fully automated system is a combination of a furnace and a VPS system. Unique, 120 kVA vacuum plasma spray forming system.
The chamber temperature (hence substrate) can be maintained at up to 800 C.
The HVOF spray booth includes an Diamond Jet HVOF gun which is connected to a three degrees of freedom robot.
Wire Arc Spray
In the wire arc spraying process, electrical energy is used to create an arc that can heat and melt tips of two consumable wires, from which the molten materials are sprayed as a high temperature/high velocity flame. A cross flow stream of gas is also present to ease the detachment of the molten material from both cathode and anode. Spraying system that is being used in the lab is VALUARC 300 manufactured by Sulzer-Metco. The characterizing parameters of the process that define the state of coating are the material of the wire (Aluminum wires are mainly used), voltage that is applied on the wires (20 to 40V), wire feed rate (5 to 10 m/min), and pressure of the atomizing gas (30 to 70psi).
RF-ICP (Radio-Frequency Inductively-Coupled Plasma) is formed within a quartz tube by applying a radio frequency current across a load coil wound around the tube. The plasma is produced by a RF plasma system consisting of the plasma torch (Precision Glassblowing, Centennial, CO) equipped with the 40 MHz RF generator and power supply (Advanced Energy Industries Inc.). The plasma was operated at the input power of 0.1 ~ 1 kW and under atmospheric pressure. The working gas was pure argon.
Hybrid DC/RF-ICP Plasma Spray
The reactor includes a hybrid DC/RF-ICP plasma torch issuing into a vacuum chamber that contains a computer driven 2-axis plus rotation substrate manipulator. Reactor pressure can be controlled and maintained at a selected level. The hybrid plasma torch has the advantages of both dc plasma and RF-ICP. More importantly, it does not suffer from the shortcomings of either device. In the case of the RF-ICP, the problem of rapid cool down of the jet and the lack of heating along the axis is solved by introduction of the DC plasma jet.
The impact of a liquid droplet on a solid surface is a fascinating phenomenon. There has been much effort devoted to studies of droplet impact, motivated by the development of several technologies that involve deposition of liquid droplets on solid surfaces. Applications in which droplet impact models have been used include spray cooling of hot surfaces, ink jet printing, spray painting, fire suppression using sprinkler systems, spray forming, deposition of solder bumps on printed circuit boards, thermal spray coating, soil erosion by rain drops, and ice accumulation on electric wires and aircraft. From a researcher's viewpoint, analysis of droplet impact and splashing offers very interesting challenges. Much of the physical phenomena involved is poorly understood, including flow of free liquid surfaces, motion of a liquid-solid-air contact line, wetting of solid surfaces, and fluid instabilities that cause formation of fingers around the droplet periphery, leading to detachment of satellite droplets. The problem becomes even more complex if droplets freeze while spreading.
High Velocity Droplet Impact Experiments
To achieve high impact velocities, it is easier to accelerate the substrate rather than the droplet. One way of doing this is to mount the substrate on the end of a rotating arm. The schematic diagram shows the apparatus we built to capture images of droplet impact by synchronizing the ejection of a droplet from the generator with the position of the moving substrate.
The test surfaces on which droplets impinged were stainless steel coupons mounted on an aluminum plate bolted to the outer rim of a 400 mm diameter aluminum flywheel. A vertical rod inserted through the hub of the flywheel was connected to the shaft of a servo motor that could reach rotational speeds of up to 3500 rpm, giving the test surface linear velocities of up to 80 m/s.
Schematic diagram of apparatus to photograph high velocity droplet impact
Photographing plasma particle impact
Fundamental studies of plasma-spray coating processes have found that the temperature of the substrate on which molten droplets impact influences their morphology, size, and extent of splashing. Splat morphology affects coating properties such as porosity, adhesion strength, and microstructure Several investigators have found that, for plasma-sprayed particles, increasing substrate temperature reduced the occurrence of splashing and produced disk-like splats.
Photographing droplets in a plasma spray at different stages during impact gives insight into the dynamics of splat formation on both hot and cold substrates. CACT, in collaboration with the National Research Council of Canada’s Industrial Materials Institute (NRC-IMI) in Boucherville, Quebec, are conducting studies to: use a rapid CCD photograph metal and ceramic particles impacting on solid surfaces; and to use high speed two-color pyrometry to measure the temperature and size evolutions of particles during spreading.
Schematic of the experimental assembly to photograph plama particle impact
Many engineering applications require the production of small, uniform sized droplets. Droplet generators were first developed as research tools to investigate droplet dynamics, spray cooling and droplet combustion, or to calibrate particle size measuring instruments. They have since found use in many industrial applications such as ink-jet printing, dispensing controlled volumes of pharmaceuticals, and deposition of adhesive or solder droplets on circuit boards. In recent years there has been great interest in droplet based manufacturing techniques for rapid prototype production. Development of all these technologies requires a reliable method of generating droplets on demand. CACT has developed a pneumatic droplet-on-demand generator which works by applying pulses of pressurised gas to liquid contained in a chamber, forcing out droplets through a nozzle in the bottom plate of the generator. A solenoid valve is rapidly opened and closed to create pressure pulses. There are no moving parts in contact with the liquid, making the generator simple to build, sturdy, and easy to adapt to high temperature applications.
Schematic diagram of molten metal droplet generator
Rapid Prototyping by Droplet Deposition
Rapid prototyping by deposition of droplets is an additive process in which components are manufactured from molten materials in a single operation without the use of a mold or other tooling. Near net shape parts are fabricated by sequentially depositing molten droplets layer by layer. Droplet based manufacturing techniques have been successfully used to make components out of wax, polymers, and ice, which are useful for prototyping or to make patterns for castings. Various commercial versions of this technology, popularly known as 3-D printers, have been available for several years.
A logical next step in rapid prototyping technology would be to make functional metal parts by droplet deposition. However, this has proved to be far more complex than simply adapting existing 3-D printers to produce molten metal droplets. Metals of commercial importance have much higher values of surface tension, melting temperature, latent heat of solidification and thermal conductivity than waxes or polymers, making it much more difficult to make droplets fuse together to form a smooth surface after solidification. CACT has developed techniques to make three-dimensional metal parts by droplet deposition. A pneumatic droplet generator is used to produce droplets, which are deposited on a substrate mounted on a programmable stage. By manipulating the substrate complex shapes can be built up.
Three-dimensional objects fabricated by depositing 0.18 mm diameter tin droplets