Thick coatings deposited by Thermal Spray

Thick coatings deposited by Thermal Spray

Bulk requirements: structural properties, low cost Surface requirements: resistance to wear, oxidation, corrosion, A mechanical resistant and cheap material for the bulk Surface Engineering approach A new problem A material able to withstand aggressive environments for the surface The adhesion of the coating material to the substrate, during deposition and during the operative life of the component N 2

The term Thermal spray is used to indicate a family of deposition technologies, which allow to obtain a layer on the substrate surface from a liquid spray of the coating material. Thermal spraying was invented by the Swiss engineer Ulrich Schoop. His first patent was deposited in 1909. N 3

The baseline: Flame Spray The coating material, in form of powder, is injected in a flame. Powder grains are melted and accelerated toward the substrate, where they flatten and solidify. N 4

Morfology of particles on impact «Cake» structure «Flower» structure N 5

Variants of Thermal spraying Baseline Flame Spray Higher temperature Plasma Spray Higher velocity HVOF, Cold Spray Different concepts Arc Spray, PTA N 6

Plasma spray N 7

Plasma spray variants APS (Atmospheric Plasma Spray) VPS (Vacuum Plasma Spray) IPS (Inert gas Plasma Spray N 8

HVOF (High Velocity Oxy Fuel) N 9

Cold Spray N 10

Arc Spray N 11

Plasma Transferred Arc N 12

Comparison among the processes Flame/gas velocity (m/s) Flame/gas temperature (K) Coating quality Coating cost Flame Spray 80-100 3.000 Low Low HVOF 1.000 3.400 High Medium Plasma spray 200-400 15.000 High High N 13

Materials which can be deposited by thermal spray: All the materials which do not decompose before melting. N 14

Materials which can be deposited by the different variants of thermal spray Flame Spray HVOF Plasma spray Arc Spray All materials with melting point lower than 3000 K Stainless steels and superalloys CerMet (ceramic particles dispersed in a metallic matrix): e.g., WC C, Cr 3 C 2 NiCr All materials presenting a stable molten phase: All the materials which can be deposited by the other thermal spray variants Refractory metals (W, Ta, Mo) High melting point ceramics (ZrO 2 ) Low melting metals (Al, Zn) Stainless steels and superalloys Composites (ceramic particles dispersed in a metallic matrix) N 15

Characteristics of the coatings deposited by the different variants of thermal spray Flame Spray HVOF Plasma spray Typical thickness a few hundreds of µm Typical adhesion strength 15 20 MPa High porosity content (> 10%) Thicknesses from a few hundreds of µm up to a few mm Typical adhesion strength 35 45 MPa Very low porosity content (< 2%) Typical thickness a few hundreds of µm Typical adhesion strength 20 30 MPa for APS Coatings, more than 100 MPa for heat treated VPS coatings Intermediate porosity content (5 8 %) N 16

Thermal spray is used in a wide range of applications. Automotive: Aviation: Oil & Gas: wear protection of cylinder liners, piston rings, fuel injectors. abradable coatings, wear protective coatings, thermal barrier coatings on gas turbines; wear protection of landing gears. erosion-corrosion protection of drilling equipment; protection of component from sand erosion; coating of valves. Power generation: coatings on gas turbines; erosion-corrosion protection of boilers in power production plants from Biomass/Waste. Paper industry: abrasive and sliding wear protection of calendar rolls. N 17

A typical example of application: TBC (thermal Barrier Coatings) for gas turbines N 18

The efficiency of a turbine depends on the gas inlet temperature. At the time such a temperature is around 1400 C, and the trend is to raise it further. The components more exposed to the hot gas stream are the combustion chamber and the first stage blades and vanes. N 19

The components for the hot working parts are made of Ni-based superalloys, with a maximum working temperature of 900 C. To enable them to withstand the hot gases, such components are cooled and coated with a ceramic layer with low thermal conductivity. N 20

The protection of the hot working parts is obtained by a complex system, composed by a two layered coating deposited by plasma spray. The first layer is metallic, and provides the oxidation resistance; the second layer is ceramic, and provides the thermal insulation. N 21

An important topic: hard chromium coatings replacements Hard chromium coatings are still widely used to protect mechanical components against wear. They are obtained by an electro-chemical deposition process. During the process, highly toxic Cr +6 is generated. N 22

Among the other options for hard chromium replacement, carbide based coatings deposited by HVOF have been successfully tested and are now employed in several applications. In particular, hydraulic cylinders and seals of landing gears are currently protected by tungsten carbide based coatings. N 23

Coatings for the future: UHTC (Ultra High Temperature Ceramic) for space vehicles Next generation of orbital re-entry vehicles, as well as future hypersonic airplanes, will be likely to include sharp-shaped structures as wing leading edges and nose-caps. This poses the attention on the needs for materials capable to operate at very high temperatures, presumably greater that 2000 C. N 24

UHTC materials Ultra High Temperatures Ceramics Diborides High melting point (>3000 K) High thermal (and electric) conductivity High emissivity Low oxidation resistance Carbides High oxidation resistance UHTC: M-diboride + SiC (M = Ti, Hf, Zr) Since 2001, a research team composed by CSM (Centro Sviluppo Materiali) and Sapienza University of Rome is developing an innovative, proprietary methodology ( * ) to produce, by plasma spraying deposition, coatings of UHTC, to protect hot structures against high temperature oxidation. (*) Tului M., Valente T., «Process for the manufacturing of ceramic matrix composite layers and related composite materials» (2002) US patent 2002/0151427 N 25

CIRA is developing a flight demonstrator of a reentry vehicle. A prototype of the nose, made in UHTC, was realised and tested. N 26

Three test articles, simulating the nose of a new generation orbital reentry vehicle, were manufactured and tested. Nose_0 was constituted by a 150 mm high graphite cone, covered with a thick layer of C/SiC ceramic, which surface was coated with thin UHTC plasma sprayed coating. Nose_1 and Nose_2 were hybrid structure made of a massive TIP in UHTC, joined to a larger structure in C/SiC, and coated with a thick layer in UHTC. N 27

Several tests were carried out on the three test articles in various conditions. Heat fluxes up to 4.5 MW/m2 were used. The maximum temperature measured, by infra-red thermography, on the coated parts was 2073 K. No damage of the coatings (cracking, spalling, etc.) was ever observed after all the tests carried out. N 28