CA2053188C - Power feed composition for forming refractory oxide thermal shock resis tant coating, process and article - Google Patents

Abstract

A thermal spray powder feed composition for forming a refractory oxide coating comprising a mixture of zirconium silicate and zirconia stabilized with a stabilizing oxide. The as-deposited coating has a chemical composition of which by x-ray phase analysis comprises ZrSiO4 and ZrO2.x where x is selected from the group consisting of CaO, Y2O3, MgO, CeO2, and HfO2.

Description

1- 2v~ 8 POWDER FEED COMPOSI~ION FOR FORMING
REFRACTORY OXIDE THERMAL SHOCK
~ESISTANT COATI~G, PROCESS AND ARTICLE
Field of the Invention The present invention relates to a thermal spray powder feed composition for forming a refractory o~ide coating having high thermal shock resistance, increased wear resistance and resistance to spalling in thermal cycling environments and to a process for forming a refractory o~ide coating and to an article having a refractory o~ide coating.
~ackaround of the Invention This invention is related to the problem of providing a high wear and thermal shock resistant coating for hearth rolls for annealing steel, stainless steel and silicon steel sheet in a hearth (furnace). The hearth rolls carry the steel sheet through the hearth. The temperature in the hearth may vary from about 1500~ to over 20~0~F depending upon the type of steel, the travel speed of the sheet steel as it passes through the furnace and the duration of time in the furnace.
A major problem encountered in the annealing operation is the transfer or pick-up of material from the steel sheet to the hearth rolls.
If pick-up occurs, it will accumulate on the hearth rolls and damage the steel sheet being processed.
To avoid this problem frequent roll changes are required with concomitsnt costs fos replacement and lost production. This problem has become more severe in recent years since thinner sheets are ', ..

- 2 - 2~ 8 being used, along with higher ~peeds and temperature to increase productivity.
To suppress the transfer of material to the hearth roll and to increase wear resistance it is desirable to coat the hearth roll with a coating composition which is substantially chemically inert at elevated temperatures. An undercoating of metal or a ceramic-metal alloy is used to prevent spalling. Spalling may also be prevented using a graded coating in which the composition of the undercoating is gradually varied from 1~0~~ alloy to 100~~ ceramic. Unfortunately, the ceramic coatings presently available usually crack in thermal cycling due to a large difference in thermal expansion between the substrate, a heat resistant alloy, and the coating. The alloy undercoat at the interface is o~idized at or above l000~C in the presence of o~ygen leading to spalling of the ceramic layer.
When a graded coating is used, the alloy component of the coating is also o~idized which, in turn, increases the volume of the coating. Upon cooling, the coating spalls due to excessive compressive stress created by the shrinkage of the substrate.
SUMMARY OF T~ INVF~TION
It was discovered in accordance with the present in~lention that a thermally sprayed coating formed from a powder feed composition of zirconium silicate and partially stabilized zirconia possesses high resistance to thermal shock in thermal cycling. The powder feed composition of the present invention produces a coating particularly useful to 2 ~ 8 protect a hearth roll in a continuous annealing line for annealing steel, stainless steel or silicon steel sheets. The powder feed composition comprises particles of zirconium silicate in a mi~ture with particles of zirconia stabilized or partially stabilized with an o~ide selected from the group consisting of Y2O3, CaO, MgO, CeO2 and HfO2. The powder feed composition is applied by a thermal spray techni~ue to produce an as-deposited coating having a composition which by ~-ray phase analysis comprises zirconia, silica and zirconium silicate.
The major component of the powder composition is partially stabilized zirconia with the zirconium silicate preferably limited to a ma2imum of 60 wt%.
For plasma spray application the powder feed composition should comprise at least 65 wt%
stabilized zirconia with the remainder substantially zirconium silicate whereas for detonation ~un application the powder feed composition should comprise at least 40 wt% stabilized zirconia and up to 60 wt% zirconium silicate. The zirconia may be either fully or partially stabilized although partially stabilized zirconia is preferred.
It was further discovered in accordance with the present invention that spalling of the coating may be prevented at elevated temperatures e~ceeding 1150~C by thermally spraying a metallic undercoat. The preferred undercoat is a cobalt based metal matris comprising Co-Cr-Al-Ta-Y with a dispersion of A12O3.

~-16458 _ ~ _ 2 ~J~ g BRIEF DESCRIPTION OF THE DRAWING
The single figure is a schematic representation of the equipment used to test the propensity of the as-deposited coating on a hearth roll for pick-up o~ me~al or metal o~ides under dynamic and static local conditions.
DESCRIPTION OF THE PREFERRED E~RODIM~T
The present invention is based upon the discovery that a starting powder feed composition consisting essentially of a mi~ture of zirconium silicate and zirconia with the zirconia being stabilized with a stabilizing o~ide such as yttria, calcia, or magnesia may be thermally sprayed to form a coating possessing the characteristic of being resistant to thermal shock and resistant to steel or steel o~ide pick-up from a continuous annealing line. Any conventional thermal spray technigue may be used to form the coating including detonation gun deposition and plasma spray deposition. The chemical composition of the thermally sprayed coating should consist of a mi~ture of at least about 40 wt%
zirconia (ZrO2), including a stabilizer for the zirconia selected from the group consisting of CaO, Y203~ MgO, CeO2 and HfO2 with the balance zirconium silicate (ZrSiO4~ and/or its decomposition products SiO2 and ZrO2. The preferred weight percent of the component o~ides in the coating is 55 to 85%
stablized ZrO2 and 15 to 45% ZrSiO4 and/or its decomposition products SiO2 and ZrO2. The optimum weight percent of the component o~ides in the coating is 70 to 85% stablized ZrO2 and 15 to 30% ZrSiO4 and/or its decomposition products. The stabilizer .
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- 5 - 2~3~3~

should be between 2 and 20 wt% ~f the zirconia component.
The coatings are preferably applied by detonation gun deposition or plasma spray deposition. A typical detonation gun consists essentially of a water-cooled barrel which is several feet long with an inside diameter of about 1 inch.
In operation, a mi~ture of o~ygen and a fuel gas, e.g., acetylene, in a specified ratio (usually about 1:1) is fed into the barrel along with a charge of coating material in powder form. Gas i5 then ignited and the detonation wave accelerates the powder to about 2400 ft./sec. (730 m/sec.) while heating the powder close to or above its melting point. After the powder exits the barrel, a pulse of nitrogen purges the barrel and readies the system for the next detonation. The cycle is then repeated many times a second.
The detonation gun deposits a circle of coating on the substrate with each detonation. The circles of coating are about 1 inch (25 mm) in diameter and a few ten thousandths of an inch (several microns) thick. Each circle of coating is composed of many overlapping microscopic thin lenticular particles or splats corresponding to the individual powder particles. The overlapping splats interlock and bond to each other and the substrate without automatically alloying at the interface thereof. The placement of the circles in the coating deposition are closely controlled to buila-up a smooth coating of uniform thickness and to minimize substrate heating.

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In the plasma arc spray process, an electric arc is established between a non-consumable electrode and a second non-consumable electrode spaced therefrom. Gas is passed in contact with the non-consumable electrode such that it contains the arc. The arc-containing gas is constricted by a nozzle and results in a high thermal content effluent. The powders used to produce the coatings are injected into the effluent nozzle and are deposited onto the surfaces to be coated. This process, which is described in U.S. Pat. No.
2,858,411, produces a deposited coating which is sound, dense and adherent to the substrate. The applied coating also consists o~ irregularly shaped microscopic splats or leaves which are interlocked and bonded to one another and also the substrate.
In general the coating composition for the plasma arc spray process will be substantially equi~alent to its corresponding starting material composition. When using the detonation gun to apply the starting material evaporation of the components may result in a significantly different ratio of constituents in the as deposited coating. Thus some change in chemistry may occur during deposition, using any thermally sprayed process. Such changes can be compensated for by adjusting the powder composition,or deposition parameters.
8ecause of the complex phase diagram for Zr-Si-O, the solidifying ZrSiO4 powder particles may contain ZrSiO~ as a crystallographic phase and/or ZrO2~SiO2 as the decomposition products of the molten ZrSiO4 in separate crystallographic phases within D-16~58 ~ 7 ~ 2~ 8 individual splats. Thus the ZrO2 and SiO2 are intimately associated within each splat which had previously been ZrSiO4 in the powder form. By ~associated" is meant the e~tremely fine and intermi~ed crystalline structure of SiO2, ZrO2 and/or ZrSiO4 crystallites within the splat.
Although the coatings of the present invention are preferably applied by detonation or plasma spray deposition, it is possible to employ other thermal spray techniques such as, for e~ample, high velocity combustion spray (including hypersonic jet spray), flame spray and so called high velocity plasma spray methods (including low pressure or vacuum spray methods). Other technigues can be employed for depositing the coatings of the present invention as will readily occur to those skilled in the art.
The thermal spray coating may be applied directly to the metal substrate. However, an undercoat compatible with the substrate and resistant to o~idation is preferred. An ùndercoat of a ceramic-metal alloy mi~ture having a cobalt based metal matrix containing alumina is preferred.
Optimum coatings are a cobalt based alloy with alumina dispersions de~cribed in U.S. Patent No.
4,124,737, the disclosure of which is herein incorporated by reference. The refractory o~ide coating reacts with the preferred undercoat to produce an impervious thin leyer of aluminum and/or zirconium o~ide phases at the interface which prevents o~idation of the undercoat as well as to provide good bonding between the undercoat and the .

D-164~8 ..

- 8 ~ 3 ~ ~ 8 ceramic layer. The impervious layer may be less than 5 microns (0.005mm) and is produced as a result of interdiffusion or o~idation in an environment at an elevated temperature in the presence of osygen. A
similar layer may form in an inert atmosphere as a result of reaction between the o~ide overcoat and the metallic undercoat.

E~am~le l To substantiate the superiority of the coating composition of the present invention, various powder mixtures containing a yttria stabilized zirconia (ZrO298% Y203) and zirconium silicate (ZrSiO~) were mechanically blended into the blend ratios identified in Table I and fed to a plasma torch in a conventional manner to produce a coating on a 304 stainless steel bar. The ceramic coating was applied on one face of the 304 stainless steel bar (2 3/4~ ~ 3/9 ~ 1/2~h) which was first coated with a detonation gun with an undercoat of 50 to 75 microns of a cobalt-based coating undercoating of 90 (Co-25Cr-7.5Al-0.8Y-lOTa) + 10% A1203. Then the ceramic coating was ground to a 100 micrometer thickness before heat cycling.
The coating specimens were heated in air to 1150~ ~o 1200~C and held for a minimum of si~ (6 hours followed by air cooling. After five (5) cycles, the specimens were water quenched on the si~th cycle. If no spalling or partial spalling occurred, the coating was described as acceptable.
The results are shown below in Table I.

9 2 ~

TARLE I
Coating Powder MixSure % Test Result Hardness No. Al~ B" Spall;ng After ~ tiV 0.3 1 100 0 No 402 2 85 ~5 11~ 587 3 75 25 No 539 4 65 35 Parti~l Spalling~ 532 Spa11 ed 482 6 25 75 Sp~lled 535 7 0 100 Spalled Before tlQ 481 P~wder A-yttria st~biliz~d zircon1a (ZrO2 . 8% Y203) ' Powder B-zirconium silicate (ZrSiO4) A Parti~ll spall;r)g indicates A lift1ng of th~ cer~m;c layer at edges.

It is apparent from Table I that for plasma torch applications spalling did not occur using a starting powder composition of greater than 65 wt.
zirconia with above about 75 wt. % zirconia being optimum.

le 2 Similar tests were conducted using a powder feed composition of calcia stabilized zirconia (ZrO2-5CaO) and zirconium silicate in a mechanical blend at the various powder ratios labeled 8 to 12 as reflected in the following Table II. The powder feed composition was applied through a detonation gun over a preferred cobalt-based metal matri~
undercoat of (Co-25Cr-7.5Al-lOTa-.08Y) +
(30% A12O3). The wear performance, thermal shock resistance and anti-pickup performance was optimum for coatings made with mi~ture number 8.

: :

~Q~ g ZrSiO4/Zr~ ~ater Ouench fro~ 900~C ~Coatinq Chenistr~ ~t%
ZrSiO4 ZrOz 8 50/SO ZO c~cles OK 43 5 56 5 9 15~85 20 c~cles OK 21 79 30/70 5 cycles 50% spall 34 66 11 70Z30 1 c~cles 50Z spall 46 5 53 5 lZ 65/lS ZO c~cles OK 45 55 '' ' ' - - ~Coatins chemistr~ - based on metallic elerent concen-ration ~easured b~ Electron ~icroprobe Anal~zer .: -- 10 --~: .

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- 11 - 2 ~t~8 Additional tests as set forth in Examples ~, 4 and 5 were conducted to substantiate the superiority of a detonation gun coating formed from a thermal spray powder feed composition comprising zirconium silicate and partially sta~ilized zirconia (2rO2-5CaO) on a hearth roll based on its anti-pickup characteristics compared to detonation gun coatings of a conventional composition of (Co-2SCr-7.5Al-~8Y-lOTa)+10% A1203, a coating from a feed composition of ZrSiO~, and bare steel. It should be noted that while a detonation gun coating of (Co-25Cr-lOTa-7.5Al-0.8Y) ~ 10% A1203 powder is a preferred undercoat for the ceramic coatings of this invention it has been used in the past as a hearth coating by itself and for comparison purposes is designated as coating number 13 in Table III.

E~ample 3 The following anti-pickup test was conducted.
Evaluation Method: A semicircular roll simulator as depicted in Figure 1 was used. The rocker 6 is rotated a~out pivot point 7 by reciprocating arms B and 9. Two coated specimens 10, 12 were mounted under the roll for simulating reciprocating sliding motion over the specimens and static reaction under load. A powdered Fe203 and/or Fe powder 14 is placed on specimen 10 for dynamic pickup evaluation and powdered Fe203 or Fe powder 16 is placed on specimen 12 for static pickup evaluation. The test measures the propensity of Fe or Fe203 to stick to surface faces A, B and C
respectively under a load of 8.Skg.

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Test Conditions Temperature: : gOOu~
~uration Time : 4 hours Atmosphere : Nitrogen 98% + Hydrogen 2%
Pickup source : ~e & ~e203 Anti-Pickup Index: is determined by the sum of the anti-pickup points (AP) for faces A, ~ and C
based on the following Table III.
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~able III
Graded ValueDe~cription Good 3 No adhesion 2 Adhe~on can be removed by 6kg/cmZ air blow 1 Adhesion can be removed by hand-rubbing Bad O Adhesion cannot be removed by the above method6 lts AP Point with AP Point with Grand Fe Powder Fe304 Powder Total Coating A B C Total A B C Sotal 13* 0 1 1 2 0 0 1 1 3 uncoate~ O O O O O O O O O
steel * Coatin~ 14 was made with a ~owder consisting of only zlrconium ~ilicate (Zr iO~,).
** Bulk Material for reference: R~frgctory Case Steel :' :

- 13 - 2 ~

~ample No. 9 ~ Wear Resistance under elevated temperature Equi~ment: High Temperature Ball on Disk Wear Tester Test Conditions:
Temperature: 900~C
Atmosphere: N2 98% + H2 2%
Ball size: 9.252mm in diameter Weight: 2 kg Sliding Speed: 0.2m/sec Track: 35 mm in diameter Total Sliding length: 720m ~all: High carbon chromium bearing steel, Disk: Coated sample Wear Rate Calculation:
The normalized wear rate is calculated by the following procedure.
a. The wear volume is calculated from area of cross section of scar measured from a profilometer chart multiplied by the circumference of the scar.
b. Wear volume is then divided by applied load and total sliding distance.
Results Coati~q Wear rate mm~k~
8 1.2 x 10-8 13 7.2 ~ 1~-8 1~ 2B0 ~ 10-8 uncoated steel 850 ~ 10-8 - ~4 - 2~3~

E~am~le No. 5 ~eat Shock Test:
1000~C - 15 minute heating from ambient to 1000~C, water quench for 15 minutes - 1 cycle The sample substrate material had a thick plate shape with a dimension of 50250~10 mm, one 50~50 side coated. The sample substrate was 304 stainless steel.
Thermal shock resistance is evaluated by counting how many cycles the coating can survive without spalling off.
S~m~le No. of CYcles 8 no spalling after 20 cycles 1~ no spalling after 20 cycles 14 spalling after 6 cycles S~ rY of Testina As is apparent from the tables and all of the above e~amples the optimum starting powder blend will necessarily vary depending upon the stabilizer and thermal spray technique used. The as-deposited coating should preferably have at least about 40 wt~
stabilized zirconia and up to 60 wt% zirconium silicate. The preferred concentration of the component o~ides in the coating independent of crystalline structure should be between about 55 to about 85 wt% stabilized ZrO2 and 15 to 45% ZrSiO4 and/or its decomposition products SiO2 and ZrO2 with 70 to 85 wt~ stabilized ZrO2 being optimum. The concentration of stabilizer in the as-deposited coating composition should be betweén 2 to 20 wtS of the zirconia component.

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Claims (8)

1. A hearth roll for use in the continuous annealing of steel comprising a roll having a metal surface coated with a wear resistant refractory oxide coating which comprises particles of zirconium silicate and/or its decomposition products SiO2 and ZrO2, in a mixture with particles of zirconia fully or partially stabilized with an oxide selected from CaO, Y2O3, MgO, CeO2 and HfO2 and wherein the coating composition comprises at least 40 wt.% of the stabilized zirconia and up to 60 wt.% zirconium silicate and/or its decomposition products SiO2 and ZrO2.

2. A roll as claimed in claim 1 wherein the particles of stabilized zirconia represents the majority ingredient of the coating composition.

3. A process for forming a wear resistant and thermal shock resistant refractory oxide coating on a metal hearth roll for use in the continuous annealing of steel, which process comprises:
formulating a powder feed composition comprising mixing powder particles of zirconium silicate with powder particles of zirconia at least partially stabilized with an oxide selected from CaO, Y2O3, MgO, CeO2 and HfO2 wherein the powder particle composition comprises at least 40 wt.% stabilized zirconia and up to 60 wt.% zirconium silicate and/or its decomposition products SiO2 and ZrO2; and thermally spraying the powder feed composition on the metal roll to form an as-deposited coating composition which, by x-ray phase analysis, comprises: ZrO2 x where x is one of the stabilizing oxides; ZrSiO4 and/or its decomposition products SiO2 and ZrO2, and SiO2.

4. A process as claimed in claim 3, wherein an undercoat of a ceramic metal alloy is deposited upon the metal hearth roll before thermally spraying the powder feed composition.

5. A process as claimed in claim 4 wherein the ceramic metal alloy undercoat is a cobalt based metal matrix comprising Co-Cr-Al-Ta-Y and Al2O3.

6. A process as claimed in claim 3 or 4, wherein the as-deposited coating is formed by feeding the powder feed composition through a plasma torch with the powder feed composition comprising at least 65 wt.% of the partially stabilized zirconia and up to 35 wt.%
zirconia silicate.

7. A process as claimed in claim 3 or 4, wherein the as-deposited coating is formed by feeding the powder feed composition through a detonation gun with the powder feed composition comprising at least 40 wt.% of the partially stabilized zirconia and up to 60 wt.%
zirconia silicate.

8. A process as claimed in claim 3 or 4, wherein the powder feed composition comprises at least 50 wt.%
partially stabilized zirconia.