HIGH-ENTROPY CARBIDE CERAMIC MATERIAL, CARBIDE CERAMIC COATING AND PREPARATION METHODS AND USE THEREOF
20230183851 · 2023-06-15
Inventors
- Hongxuan LI (Lanzhou City, CN)
- Xiaohong LIU (Lanzhou City, CN)
- Wenju Xu (Lanzhou City, CN)
- Bingsen Jia (Lanzhou City, CN)
- Li JI (Lanzhou City, CN)
- Huidi ZHOU (Lanzhou City, CN)
- Jianmin CHEN (Lanzhou City, CN)
Cpc classification
C23C14/3407
CHEMISTRY; METALLURGY
Y02E30/30
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
International classification
C23C14/00
CHEMISTRY; METALLURGY
Abstract
Disclosed are a high-entropy carbide ceramic material and a preparation method thereof, and also a ceramic coating and its preparation method and use. The high-entropy carbide ceramic material has a chemical composition of (ZrCrTiVNb)C and includes Zr, Cr, Ti, V, and Nb, with a same mole fraction of 6-10%.
Claims
1. A high-entropy carbide ceramic material, wherein the high-entropy carbide ceramic material has a chemical composition of (ZrCrTiVNb)C and comprises Zr, Cr, Ti, V and Nb, with a same mole fraction of 6-10%.
2. The high-entropy carbide ceramic material as claimed in claim 1, wherein the high-entropy carbide ceramic material presents a face-centered cubic crystal structure.
3. A method for preparing the high-entropy carbide ceramic material as claimed in claim 1, comprising step of conducting a multi-arc ion plating deposition on a surface of a substrate to obtain the high-entropy carbide ceramic material of (ZrCrTiVNb)C, wherein a reactive sputtering gas source for the multi-arc ion plating deposition comprises a carbon source gas and an inert gas, and cathode targets for the multi-arc ion plating deposition are a Zr metal target, a Cr metal target, a Ti metal target, a V metal target, and an Nb metal target.
4. The method as claimed in claim 3, wherein a flow ratio of the carbon source gas to the inert gas is in the range of (1-2): 1, and the reactive sputtering gas source has a pressure of 0.4-0.8 Pa; currents of the cathode targets each are independently in the range of 50-125 A, and a pulsed bias voltage applied is in the range of −400 V to −200 V; and the multi-arc ion plating deposition is conducted at a temperature of 300-400° C.
5. A ceramic coating, comprising a transition layer, and a carbide ceramic layer disposed on the transition layer, wherein the carbide ceramic layer is formed from the high-entropy carbide ceramic material as claimed in claim 1.
6. The ceramic coating as claimed in claim 5, wherein the carbide ceramic layer has a thickness of 2-10 μm.
7. The ceramic coating as claimed in claim 5, wherein the transition layer has a chemical composition of ZrCrTiVNb and a thickness of 200-800 nm.
8. A method for preparing the ceramic coating as claimed in claim 5, comprising step of conducting a multi-arc ion plating deposition on a surface of the transition layer to obtain the high-entropy carbide ceramic layer having a chemical composition of (ZrCrTiVNb)C, wherein a reactive sputtering gas source for the multi-arc ion plating deposition comprises a carbon source gas and an inert gas, and cathode targets for the multi-arc ion plating deposition are a Zr metal target, a Cr metal target, a Ti metal target, a V metal target, and an Nb metal target.
9. The method as claimed in claim 8, wherein the transition layer is prepared by a process comprising step of conducting a multi-arc ion plating deposition on a surface of a substrate to obtain the transition layer.
10. A method for preparing the high-entropy carbide ceramic material as claimed in claim 2, comprising step of conducting a multi-arc ion plating deposition on a surface of a substrate to obtain the high-entropy carbide ceramic material of (ZrCrTiVNb)C, wherein a reactive sputtering gas source for the multi-arc ion plating deposition comprises a carbon source gas and an inert gas, and cathode targets for the multi-arc ion plating deposition are a Zr metal target, a Cr metal target, a Ti metal target, a V metal target, and an Nb metal target.
11. A ceramic coating, comprising a transition layer, and a carbide ceramic layer disposed on the transition layer, wherein the carbide ceramic layer is formed from the high-entropy carbide ceramic material as claimed in claim 2.
12. A ceramic coating, comprising a transition layer, and a carbide ceramic layer disposed on the transition layer, wherein the carbide ceramic layer is formed from the high-entropy carbide ceramic material as prepared by the method as claimed in claim 3.
13. A ceramic coating, comprising a transition layer, and a carbide ceramic layer disposed on the transition layer, wherein the carbide ceramic layer is formed from the high-entropy carbide ceramic material as prepared by the method as claimed in claim 4.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0024]
[0025]
[0026]
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0027] The present disclosure provides a high-entropy carbide ceramic material of (ZrCrTiVNb)C, which includes Zr, Cr, Ti, V and Nb, with a same mole fraction of 6-10%.
[0028] The high-entropy carbide ceramic material described in the present disclosure has a chemical composition of (ZrCrTiVNb)C.
[0029] In the present disclosure, in the (ZrCrTiVNb)C, a mole fraction of Zr is in the range of 6-10%, and preferably 6.5-9%; a mole fraction of Cr is in the range of 6-10%, and preferably 6.5-9%; a mole fraction of Ti is in the range of 6-10%, and preferably 6.5-9%; a mole fraction of V is in the range of 6-10%, preferably 6.5-9%; a mole fraction of Nb is in the range of 6-10%, preferably 6.5-9%; the mole fractions of Zr, Cr, Ti, V and Nb are the same.
[0030] In some embodiments of the present disclosure, the high-entropy carbide ceramic material illustrated in this disclosure has a face-centered cubic crystal structure. In some embodiments, the high-entropy carbide ceramic material has a single-phase structure. In some embodiments, the high-entropy carbide ceramic material according to the present disclosure is of a single face-centered cubic (FCC) crystal structure. The FCC crystal structure is a solid-solution single-phase crystal structure, and has high-entropy characteristics. Therefore, the high-entropy carbide ceramic material according to the present disclosure, preferably with the FCC crystal structure, has a typical high-entropy structure, and has high-entropy characteristics.
[0031] The present disclosure provides a method for preparing the high-entropy carbide ceramic material described in the technical solutions above, which includes the following steps:
[0032] conducting a multi-arc ion plating deposition (hereinafter referred to as first deposition) on a surface of a substrate (hereinafter referred to as first substrate) to obtain the high-entropy carbide ceramic material of (ZrCrTiVNb)C, wherein a reactive sputtering gas source for the multi-arc ion plating deposition includes a carbon source gas and an inert gas (hereinafter referred to as first inert gas), and cathode targets for the multi-arc ion plating deposition are a Zr metal target (hereinafter referred to as first Zr metal target), a Cr metal target (hereinafter referred to as first Cr metal target), a Ti metal target (hereinafter referred to as first Ti metal target), a V metal target (hereinafter referred to as first V metal target) and an Nb metal target (hereinafter referred to as first Nb metal target).
[0033] In some embodiments of the present disclosure, the carbon source gas is at least one selected from the group consisting of CH.sub.4 and C.sub.2H.sub.2, and preferably is C.sub.2H.sub.2.
[0034] In specific embodiments of the present disclosure, the first inert gas is Ar gas.
[0035] In some embodiments of the present disclosure, a flow ratio of the carbon source gas to the first inert gas is in the range of (1-2): 1, and preferably (1.2-1.5): 1.
[0036] In some embodiments of the present disclosure, the reactive sputtering gas source has a pressure of 0.4-0.8 Pa, and preferably 0.45-0.7 Pa.
[0037] In some embodiments of the present disclosure, the first Zr metal target has a purity of greater than or equal to 99%. In some embodiments, the first Cr metal target has a purity of greater than or equal to 99%. In some embodiments, the first Ti metal target has a purity of greater than or equal to 99%. In some embodiments, the first V metal target has a purity of greater than or equal to 99%. In some embodiments, the first Nb metal target has a purity of greater than or equal to 99%.
[0038] In the present disclosure, there is no special requirements for the sources of the first Zr metal target, the first Cr metal target, the first Ti metal target, the first V metal target, and the first Nb metal target. In some embodiments, they are purchased directly.
[0039] In some embodiments of the present disclosure, currents of the first Zr metal target, the first Cr metal target, the first Ti metal target, the first V metal target, and the first Nb metal target each are independently in the range of 50-125 A, and preferably 65-120 A.
[0040] In some embodiments of the present disclosure, a pulsed bias voltages applied is in the range of −400 V to −200 V, and preferably −250 to −250 V.
[0041] In the present disclosure, the first substrate is used as a receiving container when preparing the high-entropy carbide ceramic material by the multi-arc ion plating. In the present disclosure, there are no special requirements for what the first substrate is made of.
[0042] In some embodiments of the present disclosure, the first deposition is conducted at a temperature of 300-400° C., and preferably 320-350° C.
[0043] The present disclosure provides a ceramic coating, which includes a transition layer and a carbide ceramic layer disposed on a surface of the transition layer, wherein the carbide ceramic layer is formed from the high-entropy carbide ceramic material described in the technical solutions above or the high-entropy carbide ceramic material prepared by the method described in the technical solutions above.
[0044] In some embodiments of the present disclosure, the carbide ceramic layer has a thickness of 2-10 μm, and preferably 2.5-8 μm.
[0045] In some embodiments of the present disclosure, the transition layer has a chemical composition of ZrCrTiVNb. In some embodiments, the transition layer has a thickness of 200-800 nm, and preferably 250-700 nm.
[0046] The present disclosure provides a method for preparing the ceramic coating described in the technical solutions above, which includes the following step:
[0047] conducting a multi-arc ion plating deposition on a surface of the transition layer to obtain the high-entropy carbide ceramic layer of (ZrCrTiVNb)C, wherein a reactive sputtering gas source for multi-arc ion plating deposition includes a carbon source gas and an inert gas, and cathode targets for the multi-arc ion plating deposition are a Zr metal target, a Cr metal target, a Ti metal target, a V metal target, and an Nb metal target.
[0048] In some embodiments of the present disclosure, the transition layer is prepared by a process, which includes the following step: conducting a multi-arc ion plating deposition (hereinafter referred to as second deposition) on a surface of a substrate (hereinafter referred to as second substrate) to obtain the transition layer.
[0049] In the present disclosure, the transition layer has a chemical composition of ZrCrTiVNb. In some embodiments, the multi-arc ion plating deposition is conducted in an inert gas atmosphere (hereinafter referred to as second inert gas). In some embodiments, cathode targets are a Zr metal target (hereinafter referred to as second Zr metal target), a Cr metal target (hereinafter referred to as second Cr metal target), a Ti metal target (hereinafter referred to as second Ti metal target), a V metal target (hereinafter referred to as second V metal target), and an Nb metal target (hereinafter referred to as second Nb metal target).
[0050] In the present disclosure, there are no special requirements for the second substrate. In some embodiments of the present disclosure, the second substrate is made of a metal. In specific examples of the present disclosure, the second substrate is made of one selected from the group consisting of a GH4169 high-temperature nickel-based alloy, a 316L stainless steel, and an M2 high-speed steel.
[0051] In some embodiments of the present disclosure, before the second deposition of the transition layer, the method according to the present disclosure further includes sequentially subjecting the second substrate to a solvent cleaning and an activation cleaning.
[0052] In some embodiments of the present disclosure, the solvent cleaning includes cleaning the second substrate with an organic solvent. In specific examples of the present disclosure, the organic solvent is acetone. In some embodiments of the present disclosure, the cleaning is conducted under an ultrasonic condition. In some embodiments of the present disclosure, the cleaning is conducted for 20 min. In the present disclosure, there are no special requirements for a specific ultrasonic implementation process.
[0053] In the present disclosure, the activation cleaning is conducted on the second substrate after the solvent cleaning. In some embodiments of the present disclosure, the activation cleaning is conducted by a multi-arc ion plating system.
[0054] In some embodiments of the present disclosure, the activation cleaning is conducted in an inert gas (hereinafter referred to as third inert gas) atmosphere. In some embodiments of the present disclosure, the third inert gas is Ar gas.
[0055] In some embodiments of the present disclosure, the third inert gas has a pressure of 1-2 Pa. In some embodiments of the present disclosure, a pulsed bias voltage for the activation cleaning is in the range of of 600-1,000 V, and preferably 650-850 V. In some embodiments of the present disclosure, the activation cleaning is conducted for 30 min.
[0056] In some embodiments of the present disclosure, the activation cleaning is conducted in a multi-arc ion plating vacuum chamber. In some embodiments of the present disclosure, before the aeration of inert gas, the vacuum chamber of multi-arc ion plating is pumped to a vacuum degree less than or equal to 6.0×10.sup.−3 Pa.
[0057] In some embodiments of the present disclosure, the activation cleaning of substrate is conducted by plasma generated from glow discharging of inert gas.
[0058] In specific examples of the present disclosure, the second inert gas is Ar gas.
[0059] In some embodiments of the present disclosure, the second inert gas has a pressure of 0.4-0.8 Pa, and preferably 0.45-0.7 Pa.
[0060] In some embodiments of the present disclosure, the second Zr metal target has a purity of greater than or equal to 99%. In some embodiments, the second Cr metal target has a purity of greater than or equal to 99%. In some embodiments, the second Ti metal target has a purity of greater than or equal to 99%. In some embodiments, the second V metal target has a purity of greater than or equal to 99%. In some embodiments, the second Nb metal target has a purity of greater than or equal to 99%.
[0061] In the present disclosure, there is no special requirements for the second Zr metal target, the second Cr metal target, the second Ti metal target, the second V metal target, and the second Nb metal target. In some embodiments, they are purchased directly.
[0062] In some embodiments of the present disclosure, currents of the second Zr metal target, the second Cr metal target, the second Ti metal target, the second V metal target, and the second Nb metal target each are independently in the range of 50-125 A, and preferably 65-120 A.
[0063] In some embodiments of the present disclosure, a pulsed bias voltage applied is in the range of −400 V to −200 V, and preferably −250 V to −250 V.
[0064] In some embodiments of the present disclosure, the second deposition is conducted at a temperature of 300-400° C., and preferably 320-350° C.
[0065] In the present disclosure, the scope of the method for preparing the carbide ceramic layer is the same as that of the method for preparing the high-entropy carbide ceramic material described above, and will be not repeated herein.
[0066] The present disclosure provides use of the ceramic coating or the ceramic coating prepared by the method described in the technical solutions above in tools, cutters, molds, aerospace equipment, or nuclear energy equipment.
[0067] The ceramic coating according to the present disclosure has high hardness and excellent corrosion resistance and self-lubricating property.
[0068] The technical solutions of the present disclosure are clearly and completely described below in conjunction with examples of the present disclosure. Apparently, the described examples are merely some rather than all of the examples of the present disclosure. All other examples obtained by a person of ordinary skill in the art based on the examples of the present disclosure without creative efforts shall fall within the scope of the present disclosure.
Example 1
[0069] A GH4169 substrate was ultrasonically cleaned with acetone for 20 min and then put into a multi-arc ion plating vacuum chamber. The multi-arc ion plating vacuum chamber was vacuumized to a vacuum degree of 5.0×10.sup.−3 Pa, and a high-purity Ar gas was introduced therein and a gas pressure was controlled to be 1.5 Pa. A pulsed bias voltage was controlled to be 800 V, and then the GH4169 substrate is cleaned by plasma generated from glow discharging of inert gas for 30 min.
[0070] A flow of the Ar gas was controlled to adjust the gas pressure to be 0.6 Pa. The Zr metal target, Cr metal target, Ti metal target, V metal target, and Nb metal target were turn on and currents thereof were adjusted to 115 A, 75 A, 55 A, 75 A, and 125 A, respectively. A pulsed bias voltage was adjusted to −200 V. A transition layer of ZrCrTiVNb was deposited with a thickness of 400 nm.
[0071] C.sub.2H.sub.2 was introduced, a flow ratio of the C.sub.2H.sub.2 to the Ar gas was adjusted to be 1.5:1, and a gas pressure was 0.5 Pa. Currents of the Zr target, the Cr target, the Ti target, the V target, and the Nb target were controlled to be 115 A, 75 A, 55 A, 75 A, and 125 A, respectively. A pulsed bias voltage was −200 V. A high-entropy ceramic layer of (ZrCrTiVNb)C was deposited with a thickness of 4.8 μm.
[0072] The hardness of ceramic coating prepared in this example was tested by a nano-indentation method. The result shows a hardness of 30 GPa.
[0073] The crystal structure of ceramic coating prepared in this example was analyzed by X-ray diffraction characterization. The result is shown in
[0074] The ceramic coating prepared in this example was subjected to a dry friction performance test. Conditions for the test were as follows: in an atmospheric environment, at a load of 5 N and a speed of 10 cm/s, a counterpart ball of Si.sub.3N.sub.4 with a diameter φ of 6 mm. Test results are shown in
[0075] The ceramic coating prepared in this example was subjected to a neutral salt spray test according to the method specified in the national standard 10125-2012, “Corrosion Tests in Artificial Atmospheres—Salt Spray Tests”. The test was specifically as follows. A sodium chloride solution with a pH of 6.5 was prepared and then sprayed into a closed salt spray test box through a spray device. A sample was put in the salt spray test box and intermittently observed whether a surface of the sample was rusted. After 1,000 h. the sample was taken out and observed whether the surface of the sample was rusted. Test results are shown in
Example 2
[0076] A 316 stainless steel substrate was ultrasonically cleaned with acetone for 20 min and then put into a multi-arc ion plating vacuum chamber. The multi-arc ion plating vacuum chamber was vacuumized to a vacuum degree of 5.0×10.sup.−3 Pa, and a high-purity Ar gas was introduced therein and a gas pressure was controlled to be 2 Pa. A pulsed bias voltage was controlled to be 600 V, and then the 316 stainless steel substrate is cleaned by plasma generated from glow discharging of inert gas for 30 min.
[0077] A flow of the Ar gas was controlled to adjust the gas pressure to be 0.8 Pa. A Zr metal target, a Cr metal target, a Ti metal target, a V metal target, and an Nb metal target were turn on, and currents thereof were adjusted to 120 A, 80 A, 60 A, 80 A, and 120 A, respectively. A pulsed bias voltage was adjusted to −400 V, and a transition layer of ZrCrTiVNb was deposited with a thickness of 200 nm.
[0078] C.sub.2H.sub.2 was introduced, a flow ratio of the C.sub.2H.sub.2 to the Ar gas was adjusted to be 1:1, and a gas pressure was 0.5 Pa. Currents of the Zr target, the Cr target, the Ti target, the V target, and the Nb target were controlled to be 120 A, 80 A, 60 A, 80 A, and 120 A, respectively. A pulsed bias voltage was −400 V, and a high-entropy ceramic layer of (ZrCrTiVNb)C was deposited with a thickness of 2.5 μm.
[0079] The ceramic coating prepared in this example was tested by same methods as those in Example 1. The test results show that the ceramic coating (ZrCrTiVNb)C prepared in this example has a simple FCC solid solution single-phase crystal structure, and has a hardness of 26 GPa and a friction coefficient of 0.15. After a neutral salt spray test for 1,000 h, there is no obvious corrosion spots on the surface of the ceramic coating.
Example 3
[0080] A GH4169 substrate was ultrasonically cleaned with acetone for 20 min and then put into a multi-arc ion plating vacuum chamber. The multi-arc ion plating vacuum chamber was vacuumized to a vacuum degree of 5.0×10.sup.−3 Pa, and a high-purity Ar gas was introduced therein and a gas pressure was controlled to be 1 Pa. A pulsed bias voltage was controlled to be 1,000 V, and then the GH4169 substrate is cleaned by plasma generated from glow discharging of inert gas for 30 min.
[0081] A flow of the Ar gas was controlled to adjust the gas pressure to be 0.8 Pa. The Zr metal target, a Cr metal target, a Ti metal target, a V metal target and an Nb metal target were turn on, and currents thereof were adjusted to 110 A, 70 A, 55 A, 70 A, and 110 A, respectively. A pulsed bias voltage was adjusted to −200 V, and a transition layer ZrCrTiVNb was deposited with a thickness of 700 nm.
[0082] C.sub.2H.sub.2 was introduced, a flow ratio of the C.sub.2H.sub.2 to the Ar gas was adjusted to be 1.5:1, and a gas pressure was 0.5 Pa. Currents of the Zr target, the Cr target, the Ti target, the V target, and the Nb target were controlled to be 110 A, 70 A, 55 A, 70 A, and 110 A, respectively. A pulsed bias voltage was −200 V, and a high-entropy ceramic layer (ZrCrTiVNb)C was deposited with a thickness of 8.5 μm.
[0083] The ceramic coating prepared in this example was tested by same methods as Example 1. The test results show that the ceramic coating (ZrCrTiVNb)C prepared in this example has a simple FCC solid solution single-phase crystal structure, and has a hardness of 28 GPa and a friction coefficient of 0.1. After a neutral salt spray test for 1,000 h, there is no obvious corrosion spots on the surface of the ceramic coating.
Comparative Example 1
[0084] A GH4169 substrate was ultrasonically cleaned with acetone for 20 min and then put into a multi-arc ion plating vacuum chamber. The multi-arc ion plating vacuum chamber was vacuumized to a vacuum degree of 5.0×10.sup.−3 Pa, and a high-purity Ar gas was introduced therein and a gas pressure was controlled to be 1 Pa. A pulsed bias voltage was controlled to be 1,000 V, and then the GH4169 substrate is cleaned by plasma generated from glow discharging of inert gas for 30 min.
[0085] A flow of the Ar gas was controlled to adjust the gas pressure to be 0.8 Pa. The Zr metal target, a Cr metal target, a Ti metal target, a V metal target, and an Nb metal target were turned on. Currents of the Zr target, the Cr target, the Ti target, the V target, and the Nb target were controlled to be 110 A, 70 A, 55 A, 70 A, and 110 A, respectively. A pulsed bias voltage was −200 V, and a deposition was conducted, obtaining a transition layer ZrCrTiVNb with a thickness of 700 nm.
[0086] C.sub.2H.sub.2 was introduced, a flow ratio of the C.sub.2H.sub.2 to the Ar gas was adjusted to be 0.5:1, and a gas pressure was 0.5 Pa. Currents of the Zr target, the Cr target, the Ti target, the V target, and the Nb target were controlled to be 110 A, 70 A, 55 A, 70 A, and 110 A, respectively. A pulsed bias voltage was −200 V, and a deposition was conducted, obtaining a coating of an amorphous structure with a disordered atomic combination, rather than a high-entropy ceramic layer (ZrCrTiVNb)C.
Comparative Example 2
[0087] A GH4169 substrate was ultrasonically cleaned with acetone for 20 min and then put into a multi-arc ion plating vacuum chamber. The multi-arc ion plating vacuum chamber was vacuumized to a vacuum degree of 5.0×10.sup.−3 Pa, and a high-purity Ar gas was introduced therein and a gas pressure was controlled to be 1 Pa. A pulsed bias voltage was controlled to be 1,000 V, and the GH4169 substrate is cleaned by plasma generated from glow discharging of inert gas for 30 min.
[0088] A flow of the Ar gas was controlled to adjust the gas pressure to be 0.8 Pa. Power supplies of a Zr metal target, a Cr metal target, a Ti metal target, a V metal target, and an Nb metal target were turned on. Currents of the Zr target, the Cr target, the Ti target, the V target, and the Nb target were controlled to be 110 A, 70 A, 55 A, 70 A, and 110 A, respectively. A pulsed bias voltage was −200 V, and a deposition was conducted, obtaining a transition layer ZrCrTiVNb with a thickness of 700 nm.
[0089] C.sub.2H.sub.2 was introduced, a flow ratio of the C.sub.2H.sub.2 to the Ar gas was adjusted to be 2.5:1, and a gas pressure was 0.5 Pa. Currents of the Zr target, the Cr target, the Ti target, the V target, and the Nb target were controlled to be 110 A, 70 A, 55 A, 70 A, and 110 A, respectively. A pulsed bias voltage was −200 V, and a coating of an amorphous structure was obtained rather than a high-entropy ceramic layer (ZrCrTiVNb)C of a simple single-phase structure.
[0090] The above descriptions are merely preferred embodiments of the present disclosure. It should be noted that a person of ordinary skill in the art may further make several improvements and modifications without departing from the principle of the present disclosure, but such improvements and modifications should be deemed as falling within the scope of the present disclosure.